Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 Vanua Levu, Fiji

CHAPTER XVII

Chapter 453,990 wordsPublic domain

THE VOLCANIC ROCKS OF VANUA LEVU

THE varied character of the volcanic rocks in my collection is brought out in the following Table, where I have grouped about 400 rock-sections, excluding those of the tuffs and finer detrital deposits. The small proportion of plutonic rocks should be noted.

Olivine-basalts 23 per cent. Augite-andesites 40 " " Hypersthene-augite-andesites 17 " " Acid andesites, including hornblende and quartz-andesites, &c. 12 " " Oligoclase-trachytes, quartz-porphyries or rhyolites 6 " " Hypersthene-gabbros and diorites 2 " " ---- 100

In order to avoid the necessity of frequently describing rocks of the same type it has been found requisite to devise a method of classification. In carrying out this somewhat laborious task I have often been surprised at the readiness with which rocks, the relations of which had been previously very difficult to ascertain, fell into their place in the scheme. Although many of my uncertainties have been thus removed, a large number of doubtful points remain. I venture to think, however, that others may be able to employ and also to extend the method of classification here employed.

The general plan followed has been worked out in detail for the olivine-basalts and the pyroxene-andesites of the more basic type; whilst lack of materials has prevented its further elaboration in the case of the acid andesites, oligoclase-trachytes, quartz-porphyries, &c. The treatment of all the classes has been uniform, the scheme being the same whether applied to a basalt or to a dacite.

In describing the general method I will take the Augite class. As will be seen in the Synopsis that follows, this class is first divided according to the absence or presence of a groundmass into two sub-classes, the first referring to the plutonic rocks, which, however, are not represented in my collection, the second comprising the augite-andesites which make up 40 per cent. of the total. These andesites are again divided into four orders, according as the groundmass presents parallel or non-parallel felspar-lathes, or short and stout felspars (orthophyric), or displays a felsitic character. The last two orders are practically unrepresented here, though many examples of them are found amongst the more acid andesites. Each order is then split up into three sub-orders depending on the nature of the pyroxene of the groundmass, whether granular, prismatic, or ophitic.

Each sub-order is broken up into two sections, one displaying plagioclase-phenocrysts, the other without them, or possessing very few of them. The first section is divided into two genera, according to the character of the plagioclase-phenocrysts, whether glassy or opaque, the second genus often comprising rocks allied to the porphyrites. The second or aphanitic section is subdivided into two genera according to the character of the pyroxene-phenocrysts; in the one case they are macroporphyritic; in the other they are either small or absent. The genera are split into four species according to the length of the felspar-lathes, a method which readily separates out the doleritic rocks. In cases where the materials are abundant, the genera have been first divided into porphyritic and non-porphyritic sub-genera, based on the macroporphyritic or the micro-porphyritic character of the plagioclase-phenocrysts, when present. The species can be also split into sub-species, according to the degree of basicity of the rocks, as indicated by the specific gravity.

This method is fully worked out in the later pages and need not be further described here. With abundant material from different regions it appears to me that a ready mode is here afforded of assigning to a rock its place in the scheme. In this way it would be possible to follow the systematist in his method of comparing plants and animals from different localities. To facilitate this end, I have suggested in the synopsis the employment of abbreviations, so that the description of the critical characters of a rock can be condensed into a formula capable of easy interpretation.

As an example of the use of these abbreviations I will take the instance of a common form of augite-andesite which is represented by the formula:—“_Plag, aug, matr, flu, gran, non-phen, parv, ·1-·2 mm._” This is the formula for an aphanitic augite-andesite, and it signifies that it is a rock of the plagioclase-augite class possessing a groundmass showing parallel felspar-lathes (between ·1 and ·2 mm, in average length) and granular pyroxene, and displaying no phenocrysts of plagioclase or only a very few of small size, whilst pyroxene phenocrysts if present are micro-porphyritic.

As another example the following formula for a type of porphyrite found in this island may be given:—_Plag, hypersth-aug, matr, orth, prism, phen, opac_. This is an andesite in which rhombic and monoclinic pyroxene are associated both in the phenocrysts and in the groundmass where the pyroxene is prismatic and not granular. The plagioclase phenocrysts are opaque and the felspars of the groundmass are of the orthophyric type.

There are one or two points that require further reference. In the first place the early employment in the scheme of the characters of the felspars of the groundmass for distinguishing the orders scarcely seems needed in the cases where they take the lathe-form; but the importance of its early use is shown in the acid andesites where it is certainly of prime importance in an early stage of the classification to distinguish the rocks by the character of the felspars of the groundmass, whether lathe-like, orthophyric, or felsitic. It may also be objected that the two orders obtained by dividing the lathe group into “parallel” and “non-parallel” divisions are not equivalents of the two other orders, the orthophyric and the felsitic. The distinction, however, between the flow or non-flow arrangement, though in practice not always readily established, is a far-reaching one. On _à priori_ grounds the first division might be expected to have no plutonic equivalent; whilst in the second division, it is easy to trace the gradations through the doleritic stage, where the felspar-lathes are very large, to the granitoid condition. Then, again, the ophitic habit is as a general rule confined to rocks with a doleritic or semi-doleritic groundmass, where the felspar-lathes are coarse and form a mesh-work. Two quite distinct lines of development unite in the felspar-lathes and begin to diverge with the difference in their arrangement in the groundmass.

The nature of the difference between the flow and non-flow arrangement of the felspar-lathes is well brought out in some dykes of basalt and augite-andesite that I examined in this island and also in the Valle del Bove on the Etna slopes. In the outer vitreous portion the felspar-lathes, which are fairly well represented, are all about the same length and are more or less parallel with the sides of the dyke. In the central more crystalline portion two sets of lathes can be distinguished, one (_A_) corresponding in the average length and in the flow-arrangement with the lathes of the borders, the other (_B_) being about half the length and forming a plexus between the larger parallel lathes. Those of the _A_ set, which are those that usually catch the eye in a section, are contemporaneous in their origin with those in the margins of the dyke; whilst those of the _B_ set have been subsequently formed. In the preliminary “stiffening” of the first stage of consolidation, the whole mass of the dyke would be affected. To this stage the lathes of the _A_ set belong; whilst to the later stage of consolidation which would proceed much more slowly in the interior than at the margins of the dyke, the lathes of the _B_ set are to be referred. This distinction so plainly illustrated in a dyke must be postulated for all intrusive masses; but I have not yet found it possible to make much use of it. Much ground will have first to be cleared before it can be safely employed, since it is apparent, for instance, that there are often all gradations in a slide between a lathe and a phenocryst, and that the term “phenocryst” is applied to crystals having very different histories.

With regard to the ophitic habit of some of the basaltic rocks the following conclusions may be drawn:

(_a_) Typical ophitic “plates” are not very common in the slides. More frequently the habit of the pyroxene is semi-ophitic.

(_b_) This character is as a general rule associated with the plexus or non-fluidal arrangement of the felspar lathes.

(_c_) The felspar-lathes are nearly always large, frequently averaging more than ·2 mm. in length. This coarse doleritic groundmass is almost diagnostic of an ophitic rock.

* * * * *

It is not always possible to allow for the influence of locality in drawing up such a classification as this, since it is well known that in each volcanic region the rocks have a particular facies recognisable in hand-specimens as well as in the slide, though it is not easy to express such a distinction in a definition. Perhaps this is represented in “adaptation” as we find it in the organic world, and the question arises as to the value of such characters for critical purposes. Regional variation plays such an important part that it cannot be ignored in rock-classification.

SYNOPSIS

I. CLASS.—OLIVINE ROCKS (_Plag., oliv._).

+----------------+------------------+---------------------------------+ | SUB-CLASS. | DIVISION. | ORDER.[108] | +----------------+------------------+---------------------------------+ | 1. | | | | No groundmass | | | | (_non-matr._) | | | | LIMBURGITES, | |(Not represented in collection.) | | PERIDOTITES, | | | | OLIVINE | | | | GABBROS, | | | | etc. | | { 1. | | | | { Felspar lathes not | | |{ 1. | { in flow-arrangement | | |{Olivine abundant | { (_non-flu._). | | |{ (_cop._) | { 2. | | |{ | { Felspar lathes | | 2. |{ | { in flow-arrangement | | Groundmass |{ | { (_flu._). | | (_matr._) |{ | | | OLIVINE-BASALTS|{ | { 3. | | |{ | { Felspar lathes not | | |{ 2. | { in flow-arrangement | | |{Olivine | { (_non-flu._). | | |{relatively | { 4. | | |{scanty | { Felspar lathes | | |{(_pauc._). | { in flow-arrangement | | | | { (_flu._). | +----------------+------------------+---------------------------------+

II. CLASS.—AUGITE ROCKS (_Plag., aug._).

+----------------------+----------------------------------------------+ | SUB-CLASS. | ORDER. | +----------------------+----------------------------------------------+ | | | | 1. | | | No groundmass | Not represented in collection. | | (_non-matr._) | | | GABBROS, in part. | | | |{ 1. | | |{ Felspar lathes not in flow-arrangement | | |{ (_non-flu._). | | 2. |{ | | Groundmass (_matr._) |{ 2. | | BASALTIC ANDESITES; |{ Felspar lathes in flow-arrangement | | AUGITE-ANDESITES; |{ (_flu._). | | PYROXENE-ANDESITES |{ 3. | | in part. |{ Felspars short and broad (orthophyric, | | |{ _orth._).[109] | | |{ 4. | | |{ Groundmass felsitic (_fels._)[109] | +----------------------+----------------------------------------------+

III. CLASS.—HYPERSTHENE-AUGITE ROCKS (_Plag., hypersth.-aug._).

+----------------------------+---------------------------------------+ | SUB-CLASS. | ORDER. | +----------------------------+---------------------------------------+ | 1. | | |No groundmass (_non-matr._) |Represented by the Hypersthene-gabbros | | GABBROS, in part. | in collection. | | | | | |{ 1. | | |{Felspar lathes not in flow-arrangement| | |{ (_non-flu._). | | |{ | | |{ 2. | | 2. |{Felspar lathes in flow-arrangement | | Groundmass (_matr._) |{ (_flu._). | |PYROXENE-ANDESITES, in part.|{ | | |{ 3. | | |{Felspars of the groundmass short and | | |{ broad (orthophyric, _orth._). | | |{ | | |{ 4. | | |{Groundmass granular or presenting a | | |{ mosaic (felsitic, _fels._). | +----------------------------+---------------------------------------+

IV. CLASS.—HYPERSTHENE ROCKS. V. CLASS.—HORNBLENDE-HYPERSTHENE ROCKS. VI. CLASS.—QUARTZ-HORNBLENDE-HYPERSTHENE ROCKS OR DACITES.

These last three classes are merely provisional. They include the Acid Andesites of Vanua Levu, which are all characterised by the prevalence of rhombic pyroxene amongst the phenocrysts and by its predominance or rather by its usually exclusive occurrence in the groundmass. All the classes are capable of being split up into two sub-classes and four orders as in the case of the third class. The characters of these rocks are given in Chapter XXI.

VII. CLASS.—HORNBLENDE ROCKS.

In this class are included those rocks where hornblende is the only ferro-magnesian mineral. It is only represented by two diorites described on page 251.

VIII. CLASS.—OLIGOCLASE-TRACHYTES. } IX. CLASS.—QUARTZ-PORPHYRIES AND RHYOLITES } Described in Chapter XXI.

FIRST ORDER OF THE OLIVINE BASALTS.

CHARACTERS.—Olivine in abundance. Felspars of the groundmass not in flow-arrangement.

FORMULA.—_Oliv., matr., cop., non-flu._

+------------+---------------+------------+------------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+------------+------------+ | | | Presence or | | | Number of | | Pyroxene | absence of | Character | Length | specimens. | | of the | plagioclase | of the | of felspar | | | groundmass.| phenocrysts. |phenocrysts.| lathes. | | +------------+---------------+------------+------------+--------------+ | | |{ 1. |{·02-·1 mm. | 1 | | | |{ Glassy |{ ·1-·2 " | 2 | | |{ 1. |{plagioclase|{ ·2-·3 " | | | |{ Plagioclase |{phenocrysts|{ ·3-·5 " | | | |{ phenocrysts |{ (_vitr._) | | | | |{ (_phen._) |{ | | | | |{ |{ 2. |{·02-·1 mm. | | | |{ |{ Opaque |{ ·1-·2 " | 1 | | |{ |{plagioclase|{ ·2-·3 " | 2 | | |{ |{phenocrysts|{ ·3-·5 " | | | |{ |{ (_opac._) | | | | 1. |{ | | | | | Granular |{ |{ 3. | | | | (_gran._) |{ |{ Large |{·02-·1 mm. | | | |{ |{phenocrysts|{ ·1-·2 " | | | |{ |{of olivine |{ ·2-·3 " | | | |{ 2. |{ and |{ ·3-·5 " | 1 | | |{ No |{ pyroxene, | | | | |{ plagioclase |{ over 2 mm.| | | | |{ phenocrysts |{ (_magn._) | | | | | |{ | | | | |{ (_non-phen._)|{ 4. | | | | | |{ Small |{·02-·1 mm. | | | | |{phenocrysts|{ ·1-·2 " | | | | |{of olivine |{ ·2-·3 " | | | | |{ and |{ ·3-·5 " | 1 | | | |{ pyroxene | | | | | |{under 2 mm.| | | | | |{ Pyroxene | | | | | |{scanty and | | | | | |{ often | | | | | |{ absent | | | | | |{ (_parv._) | | | | | | | | | | | |{5. _Vitr._ |} | | | |{ 3. _Phen._ |{ |} | | | 2. |{ |{6. _Opac._ |} | | | Prismatic |{ | |} |{ Not | | (_prism._) |{ |{7. _Magn._ |} |{ represented.| | |{4. _Non-phen._|{ |} | | | | |{8. _Parv._ |} | | | | | | | | | | |{9. _Vitr._ |} | | | |{ 5. _Phen._ |{ |} |{ Not | | 3. |{ |{10. _Opac._|} |{ represented.| | Ophitic |{ | | | | | (_oph._) |{ |{11. _Magn._| |{ Not | | |{6. _Non-phen._|{ | |{ represented.| | | |{12. _Parv._| ·1-·2 mm. | 1 | +------------+---------------+------------+------------+--------------+

SECOND ORDER OF THE OLIVINE-BASALTS.

CHARACTERS.—Olivine in abundance. Felspars of the groundmass in flow-arrangement.

FORMULA.—_Oliv., matr., cop., flu._

+------------+---------------+------------+------------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+------------+------------+ | | | Presence or | | | Number of | | Pyroxene | absence of | Character | Length | specimens. | | of the | plagioclase | of the | of felspar | | | groundmass.| phenocrysts. |phenocrysts.| lathes. | | +------------+---------------+------------+------------+--------------+ | | |{ 13. |{·02-·1 mm. | 3 | | | |{ Glassy |{ ·1-·2 " | 2 | | | |{plagioclase|{ ·2-·3 " | 1 | | |{ 7. |{phenocrysts|{ ·3-·5 " | | | |{ Plagioclase |{ (_vitr._) | | | | |{ phenocrysts |{ | | | | |{ (_phen._) |{ 14. |{·02-·1 mm. | | | |{ |{ Opaque |{ ·1-·2 " | 3 | | |{ |{plagioclase|{ ·2-·3 " | | | |{ |{phenocrysts|{ ·3-·5 " | | | |{ |{ (_opac._) | | | | 4. |{ | | | | | Granular |{ |{ 15. | | | | (_gran._) |{ |{ Large |{·02-·1 mm. | 2 | | |{ |{phenocrysts|{ ·1-·2 " | | | |{ |{of olivine |{ ·2-·3 " | | | |{ |{ and |{ ·3-·5 " | | | |{ |{ pyroxene, | | | | |{ |{ over 2 mm.| | | | | |{ (_magn._) | | | | |{ 8. |{ | | | | |{ No |{ 16. |} | | | |{ plagioclase |{ Small |} | | | |{ phenocrysts |{phenocrysts|} | | | |{ (_non-phen._)|{of olivine |}·02-·1 mm. | 1 | | | |{ and |} ·1-·2 " | 5 | | | |{ pyroxene, |} ·2-·3 " | 2 | | | |{under 2 mm.|} ·3-·5 " | 1 | | | |{ Pyroxene |} | | | | |{ scanty and|} | | | | |{ often |} | | | | |{ absent | | | | | |{ (_parv._) | | | | | | | | | | | |{17. _Vitr._|} | | | |{ 9. _Phen._ |{ |} | | | 5. |{ |{18. _Opac._|} |{ Not | | Prismatic |{ | |} |{ represented.| | (_prism_.) |{ |{19. _Magn._|} | | | |{ 10. |{ |} | | | |{ _Non-phen._ |{20. _Parv._|} | | | | | | | | | | |{21. _Vitr._|} | | | |{ 11. _Phen._ |{ |} | | | 6. |{ |{22. _Opac._|} |{ Not | | Ophitic |{ | |} |{ represented.| | (_oph._) |{ |{23. _Magn._|} | | | |{ 12. |{ |} | | | |{ _Non-phen._ |{24. _Parv._|} | | +------------+---------------+------------+------------+--------------+

THIRD ORDER OF THE OLIVINE-BASALTS.

CHARACTERS.—Olivine scanty. Felspars of the groundmass not in flow-arrangement.

FORMULA.—_Oliv., matr., pauc., non-flu._

+------------+---------------+------------+------------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+------------+------------+ | | | Presence or | | Length | Number of | | Pyroxene | absence of | Character | of felspar | specimens. | | of the | plagioclase | of the | lathes and | | | groundmass.| phenocrysts. |phenocrysts.| prisms. | | +------------+---------------+------------+------------+--------------+ | | |{ 25. |{·02-·1 mm. | 1 | | | |{ Glassy |{ ·1-·2 " | 13 | | |{ 13. |{plagioclase|{ ·2-·3 " | 5 | | |{ Plagioclase |{phenocrysts|{ ·3-·5 " | | | |{ phenocrysts |{ (_vitr._) | | | | |{ (_phen._) |{ | | | | |{ |{ 26. |{·02-·1 mm. | | | |{ |{ Opaque |{ ·1-·2 " | 4 | | |{ |{plagioclase|{ ·2-·3 " | | | |{ |{phenocrysts|{ ·3-·5 " | | | 7. |{ |{ (_opac._) | | | | Granular |{ | | | | | (_gran._) |{ |{ 27. | | | | |{ |{ Large |{·02-·1 mm. |} | | |{ |{phenocrysts|{ ·1-·2 " |} Not | | |{ |{of olivine |{ ·2-·3 " |} represented.| | |{ |{ and |{ ·3-·5 " |} | | |{ 14. |{ pyroxene, | | | | |{ No |{ over 2 mm.| | | | |{ plagioclase |{ (_magn._) | | | | |{ phenocrysts |{ | | | | |{ (_non-phen._)|{ 28. | | | | | |{ Small | | | | | |{phenocrysts|{·02-·1 mm. |} | | | |{of olivine |{ ·1-·2 " |} Not | | | |{ and |{ ·2-·3 " |} represented.| | | |{ sometimes |{ ·3-·5 " |} | | | |{ pyroxene, | | | | | |{under 2 mm.| | | | | |{ (_parv._) | | | | | | | | | | | |{29. _Vitr._|} | | | |{ 15. _Phen._ |{ |} | | | 8. |{ |{30. _Opac._|} |{ Not | | Prismatic |{ | |} |{ represented.| | (_prism_.) |{ |{31. _Magn._|} | | | |{ 16. |{ |} | | | |{ _Non-phen._ |{32. _Parv._|} | | | | | | | | | | | |{·02-·1 mm. | | | | |{33. _Vitr._|{ ·1-·2 " | 3 | | |{ 17. _Phen._ |{ |{ ·2-·3 " | 1 | | 9. |{ |{ |{ ·3-·5 " | 2 | | Ophitic |{ |{ | |{ Not | | (_oph_.) |{ |{34. _Opac._| |{ represented.| | |{ | | | | | |{ |{35. _Magn._|} |{ Not | | |{ 18. |{ |} |{ represented.| | |{ _Non-phen._ |{36. _Parv._|} | | +------------+---------------+------------+------------+--------------+

FOURTH ORDER OF THE OLIVINE-BASALTS.

CHARACTERS.—Olivine scanty. Felspar-lathes of the groundmass in flow-arrangement.

FORMULA.—_Oliv., matr., pauc., flu._

+------------+---------------+------------+------------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+------------+------------+ | | | Presence or | | | | | Pyroxene | absence of | Character | Length | Number of | | of the | plagioclase | of the | of felspar | specimens. | | groundmass.| phenocrysts. |phenocrysts.| lathes. | | +------------+---------------+------------+------------+--------------+ | | |{ 37. |{·02-·1 mm. | 2 | | | |{ Glassy |{ ·1-·2 " | 12 | | | |{plagioclase|{ ·2-·3 " | 6 | | |{ 19. |{phenocrysts|{ ·3-·5 " | | | |{ Plagioclase |{ (_vitr._) | | | | |{ phenocrysts |{ | | | | |{ (_phen._) |{ 38. |{·02-·1 mm. | | | |{ |{ Opaque |{ ·1-·2 " | 2 | | |{ |{plagioclase|{ ·2-·3 " | | | |{ |{phenocrysts|{ ·3-·5 " | | | 10. |{ |{ (_opac._) | | | | Granular |{ | | | | | (_gran._) |{ |{ 39. | | | | |{ |{ Large |{·02-·1 mm. |} | | |{ |{phenocrysts|{ ·1-·2 " |} Not | | |{ |{of olivine |{ ·2-·3 " |} represented.| | |{ |{ and |{ ·3-·5 " |} | | |{ |{ pyroxene, | | | | |{ 20. |{over 2 mm. | | | | |{ No |{ (_magn._) | | | | |{ plagioclase |{ | | | | |{ phenocrysts |{ 40. | | | | |{ (_non-phen._)|{ Small | | | | | |{phenocrysts|{·02-·1 mm. | | | | |{of olivine |{ ·1-·2 " | | | | |{ and |{ ·2-·3 " | 2 | | | |{ sometimes|{ ·3-·5 " | 1 | | | |{ pyroxene, | | | | | |{under 2 mm.| | | | | |{ (_parv._) | | | | | | | | | | | |{41. _Vitr._|} | | | |{ 21. _Phen._ |{ |} |{ Not | | 11. |{ |{42. _Opac._|} |{ represented.| | Prismatic |{ | | | | | (_prism._) |{ |{43. _Magn._| |{ Not | | |{ 22. |{ | |{ represented.| | |{ _Non-phen._ |{44. _Parv._| ·02-·1 mm. | 1 | | | | | | | | 12. | | | | | | Ophitic | | | |{ Not | | (_oph._) | | | |{ represented.| +------------+---------------+------------+------------+--------------+

FIRST ORDER OF THE AUGITE-ANDESITES.

CHARACTERS.—Felspar lathes or prisms of the groundmass not in flow-arrangement.

FORMULA.—_Aug., matr., non-flu._

+------------+---------------+------------+------------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+------------+------------+ | | | Presence or | | Length of | Number of | | Pyroxene | absence of | Character | felspar | specimens. | | of the | plagioclase | of the | lathes and | | | groundmass.| phenocrysts. |phenocrysts.| prisms. | | +------------+---------------+------------+------------+--------------+ | | |{ 1. |{·02-·1 mm. | 9 | | | |{ Glassy |{ ·1-·2 " | 5 | | | |{plagioclase|{ ·2-·3 " | 4 | | |{ 1. |{phenocrysts|{ ·3-·5 " | | | |{ Plagioclase |{ (_vitr._) | | | | |{ phenocrysts |{ | | | | |{ (_phen._) |{ 2. |{·02-·1 mm. | 6 | | |{ |{ Opaque |{ ·1-·2 " | 4 | | |{ |{plagioclase|{ ·2-·3 " | 3 | | |{ |{phenocrysts|{ ·3-·5 " | 3 | | 1. |{ |{ (_opac._) | | | | Granular |{ | | | | | (_gran._) |{ |{ 3. | | | | |{ |{ Large |} | | | |{ |{phenocrysts|} |{ Not | | |{ |{of augite, |} |{ represented.| | |{ 2. |{over 2 mm. |} | | | |{ No |{ (_magn._) |} | | | |{ plagioclase |{ | | | | |{ phenocrysts |{ 4. | | | | |{ (_non-phen._)|{ Small | | | | | |{phenocrysts|{·02-·1 mm. | | | | |{of augite, |{ ·1-·2 " | 3 | | | |{ under |{ ·2-·3 " | 1 | | | |{ 2 mm., |{ ·3-·5 " | 2 | | | |{ or none | | | | | |{ (_parv._) | | | | | | |{·02-·1 mm. | 1 | | | |{5. _Vitr._ |{ ·1-·2 " | 2 | | |{ 3. _Phen._ |{ | | | | 2. |{ |{6. _Opac._ | ·1-·2 " | 2 | | Prismatic |{ | | | | | (_prism._) |{ |{7. _Magn._ |} |{ Not | | |{ 4. |{ |} |{ represented.| | |{ _Non-phen._ |{8. _Parv._ |} | | | | | | | | | | | |{·02-·1 mm. | 1 | | | | |{ ·1-·2 " | 3 | | | |{9. _Vitr._ |{ ·2-·3 " | 4 | | |{ 5. _Phen._ |{ |{ ·3-·5 " | 7 | | |{ |{ | | | | 3. |{ |{ |{ ·2-·3 mm. | 1 | | Ophitic |{ |{10. _Opac._|{ ·3-·5 " | 1 | | (_oph._) |{ | | | | | |{ |{11. _Magn._| | | | |{ 6. |{ | | | | |{ _Non-phen._ |{12. _Parv._|{ ·2-·3 mm. | 3 | | | | |{ ·3-·5 " | 3 | +------------+---------------+------------+------------+--------------+

SECOND ORDER OF THE AUGITE-ANDESITES.

CHARACTERS.—Felspar-lathes of the groundmass in flow-arrangement.

FORMULA.—_Aug., matr., flu._

+------------+---------------+-------------+-----------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+-------------+-----------+ | | | Presence or | | Length of | Number of | | Pyroxene | absence of | Character | felspar | Specimens. | | of the | plagioclase | of the | lathes and| | | groundmass.| phenocrysts. |phenocrysts. | prisms. | | +----------------------------+-------------+-----------+--------------+ | | |{ 13. |{·02-·1 mm.| 10 | | | |{ Glassy |{ ·1-·2 " | 10 | | | |{ plagioclase|{ ·2-·3 " | 4 | | |{ 7. |{ phenocrysts|{ ·3-·5 " | 1 | | |{ Plagioclase |{ (_vitr._) | | | | |{ phenocrysts |{ | | | | |{ (_phen._) |{ 14. |} | | | |{ |{ Opaque |} | | | |{ |{ phenocrysts|}·02-·1 mm.| 2 | | 4. |{ |{ of |} | | | Granular |{ |{ plagioclase|} | | |(_gran._) | |{ (_opac._) | | | | |{ | | | | | |{ |{ 15. |} | | | |{ |{ Large |} |{ Not | | |{ 8. |{phenocrysts |} |{ represented.| | |{ No (or very |{ of augite, |} | | | |{few and small)|{ over 2 mm. |} | | | |{ plagioclase |{ (_magn._) | | | | |{ phenocrysts |{ | | | | |{(_non-phen._) |{ 16. |{·02-·1 mm.| 12 | | | |{Small augite|{ ·1-·2 " | 14 | | | |{phenocrysts,|{ ·2-·3 " | 4 | | | |{ under 2 mm.|{ ·3-·5 " | 3 | | | |{ or none | | | | | |{ (_parv._) | | | | | | | | | | | |{ 17. _Vitr._| ·1-·2 mm.| 1 | | |{ 9. _Phen._ |{ | |{ Not | | |{ |{ 18. _Opac._| |{ represented.| | 5. |{ | | | | | Prismatic |{ | | |{ Not | | (_prism._) |{ |{ 19. _Magn._| |{ represented.| | |{ 10. |{ |{·02-·1 mm.| 4 | | |{ _Non-phen._ |{ 20. _Parv._|{ ·1-·2 "| 4 | | | | | | | | | |{ 21. _Vitr._| ·1-·2 mm.| 3 | | |{ 11. _Phen._ |{ | |{ Not | | 6. |{ |{ 22. _Opac._| |{ represented.| | Ophitic |{ | | | | | (_oph._) |{ |{ 23. _Magn._|} | | | |{ 12. |{ |} |{ Not | | |{ _Non-phen._ |{ 24. _Parv._|} |{ represented.| +------------+---------------+-------------+-----------+--------------+

FIRST ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Felspar-lathes not in flow-arrangement.

FORMULA.—_Hypersth.-aug., matr., non-flu._

+------------+---------------+-------------+-----------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+-------------+-----------+ | | | Presence or | | | Number of | | Pyroxene | absence of | Character | Length | specimens. | | of the | plagioclase | of the | of felspar| | | groundmass.| phenocrysts. | phenocrysts.| lathes. | | +------------+---------------+-------------+-----------+--------------+ | | |{ 1. |} | | | | |{ Glassy |}·02-·1 mm.| 12 | | | |{plagioclase |} ·1-·2 " | 4 | | | |{phenocrysts |} | | | |{ 1. |{ (_vitr._) | | | | |{ Plagioclase |{ | | | | |{ phenocrysts |{ 2. |} | | | |{ (_phen._) |{ Opaque |} |{ Not | | |{ |{phenocrysts |} |{ represented.| | |{ |{ of |} | | | 1. |{ |{plagioclase |} | | | Granular | |{ (_opac._) | | | | (_gran._) |{ | | | | | |{ |{ 3. |} | | | |{ |{ Large |} | | | |{ |{phenocrysts |} |{ Not | | |{ |{ of |} |{ represented.| | |{ |{ pyroxene, |} | | | |{ 2. |{ over 2 mm. |} | | | |{No plagioclase|{ (_magn._) | | | | |{ phenocrysts |{ | | | | |{(_non phen._) |{ 4. | | | | | |{ Small | | | | | |{ pyroxene | ·3-·5 mm.| 1 | | | |{phenocrysts,| | | | | |{under 2 mm.,| | | | | |{ or none | | | | | |{ (_parv._) | | | | | | |{·02-·1 mm.| 2 | | | |{5. _Vitr._ |{ ·1-·2 " | 2 | | |{ 3. _Phen._ |{ | | | | 2. |{ |{ |{·02-·1 mm.| 1 | | Prismatic |{ |{6. _Opac._ |{ ·1-·2 " | 1 | | (_prism._) |{ | | | | | |{ |{7. _Magn._ |} | | | |{4. _Non phen._|{ |} |{ Not | | | |{8. _Parv._ |} |{ represented.| | | | | | | | | |{ 9 } | | | | 3. | |{ 10 } | |{ Not | | Ophitic | |{ 11 } | |{ represented.| | (_oph._) | |{ 12 } | | | +------------+---------------+-------------+-----------+--------------+

SECOND ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Felspar-lathes in flow-arrangement.

FORMULA.—_Hypersth.-aug., matr., flu._

+------------+---------------+-------------+-----------+--------------+ | SUB-ORDER. | SECTION. | GENUS. | SPECIES. | | +------------+---------------+-------------+-----------+ | | | Presence or | | | Number of | | Pyroxene | absence of | Character | Length | specimens. | | of the | plagioclase | of the | of felspar| | | groundmass.| phenocrysts. | phenocrysts.| lathes. | | +------------+---------------+-------------+-----------+--------------+ | | |{ 13. |} | | | | |{ Glassy |}·02-·1 mm.| 9 | | | |{plagioclase |} ·1-·2 " | 2 | | |{ 7. |{phenocrysts |} | | | |{ Plagioclase |{ (_vitr._) |} | | | |{ phenocrysts |{ | | | | |{ (_phen._) |{ 14. |} | | | |{ |{ Opaque |} |{ Not | | |{ |{phenocrysts |} |{ represented.| | |{ |{ of |} | | | 4. |{ |{ plagioclase|} | | | Granular |{ |{ (_opac._) | | | | (_gran._) |{ | | | | | |{ |{ 15. |} | | | |{ |{ Large |} | | | |{ |{phenocrysts |} |{ Not | | |{ |{of pyroxene,|} |{ represented.| | |{ |{ over 2 mm. |} | | | |{ 8. |{ (_magn._) |} | | | |{No plagioclase|{ | | | | |{ phenocrysts |{ 16. |} | | | |{(_non-phen._) |{ Small |} | | | | |{ pyroxene |} |{ Not | | | |{phenocrysts,|} |{ represented.| | | |{under 2 mm.,|} | | | | |{ or none |} | | | | |{ (_parv._) | | | | | | | | | | | |{ 17. _Vitr._| ·02-·1 mm.| 2 | | |{ 9. _Phen._ |{ | | | | 5. |{ |{ 18. _Opac._| ·02-·1 mm.| 4 | | Prismatic |{ | | | | | (_prism._) |{ |{ 19. _Magn._| |{ Not | | |{ 10. |{ | |{ represented.| | |{ _Non-phen._ |{ 20. _Parv._|{·02-·1 mm.| 4 | | | | |{ ·1-·2 " | 2 | | | | | | | | 6. | |{ 21 } | | | | Ophitic | |{ 22 } | |{ Not | | (_oph._) | |{ 23 } | |{ represented.| | | |{ 24 } | | | +------------+---------------+-------------+-----------+--------------+

THIRD ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Felspars of the groundmass short and stout (orthophyric).

FORMULA.—_Hypersth.-aug., matr., orth._

REMARKS.—The same classification is to be employed here as in the case of the two previous orders, but as the rocks in my collection that belong to this order are not numerous (nine sections), it will be sufficient to refer to the general remarks on the order on p. 290.

FOURTH ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Groundmass presenting a rudely granular appearance or blurred mosaic (microfelsitic).

FORMULA.—_Hypersth.-aug., matr., fels._

REMARKS.—My sliced specimens (five) are too few for the elaboration of this order to which the classification employed for the other orders is scarcely applicable. This is due to the partial decomposition or imperfect development of the pyroxene of the groundmass. The general characters of the order are given on p. 291.

THE PLUTONIC ROCKS.

These rocks are very infrequent and are for the most part hypersthene-gabbros or norites, with a few representatives of diorites[110] without pyroxene. True plutonic rocks did not come under my observation in the western half of the island (west of Lambasa and Savu-savu), those of Mount Thoka-singa in the Ndrandramea district making the nearest approach (see p. 302). The localities in which they were found are below enumerated:—

THE HYPERSTHENE-GABBROS.—These rocks also contain monoclinic pyroxene, and are the plutonic equivalents of the hypersthene-augite-andesites which as a rule prevail in the localities where these rocks occur. They are usually dark grey or steel-grey in colour with a specific gravity ranging from 2·7 to 2·84 and have a granitoid aspect. The following characters are common to all the specimens.

They display a mixture of plagioclase and pyroxene, the last filling up the spaces between the felspars and apparently of later formation. The plagioclase crystals, which are 1 to 2 mm. in size, are opaque; and since they are traversed by numerous fine fissures filled with dust-like decomposition products, their appearance is often semi-saussuritic. They are much cross-macled, are at times zoned, and give lamellar extinctions of andesine-labradorite (20°-30°).... The pyroxene includes both the rhombic and monoclinic forms, the last with extinction angles of over 30°. They may be associated or may occur as separate crystals, the rhombic prevailing in the less basic and the monoclinic in the more basic rocks. The rhombic pyroxene is usually more or less converted into bastite which by further change passes into a chloritic material; whilst the augite sometimes undergoes the diallagic change resulting from schillerisation.

Some special features are presented by rocks from different localities. That from Nawi is most basic and looks like a diallage-gabbro. That from the Valanga Range (sp. gr. 2·75) contains some quartz, apparently secondary and filling up the interspaces. The rock from the Na Kula Ridge shows traces of a groundmass; but it comes near the plutonic type.

A HORNBLENDE-GABBRO.—This granitoid rock, which is from the Na Kula Ridge, has a specific gravity of 2·72. Hand-specimens display large porphyritic crystals of hornblende (7 mm. long) in a base of opaque felspar and smaller hornblende. In the slide we observe besides the large crystals of plagioclase and hornblende a little pyroxene; but the mass of the rock consists of greenish-brown hornblende, plagioclase, and some secondary quartz, forming a coarse mosaic with a “grain” of about a millimetre. The hornblende is displayed in regular hexagonal sections, markedly pleochroic, and gives extinctions up to 12°. It shows no dark resorption borders; and the larger porphyritic crystals have the same characters. Almost all the plagioclase of the rock is traversed by numerous fine fissures, and often acquires a semi-saussuritic appearance from the presence of dust-like decomposition products. The lamellar extinctions indicate andesine-labradorite. The quartz occurs mostly in nests. The pyroxene is formed of large grains of both the monoclinic and rhombic types.

THE DIORITES.—The rock forming blocks in the Vunimbua River has a specific gravity of 2·78 to 2·8. It is a pretty rock showing long black blades of hornblende, 10 mm. in length, in an opaque felspar base. In the slide the hornblende, which is dark brown and markedly pleochroic, shows six-sided sections with characteristic prismatic cleavage lines, the longitudinal sections giving extinctions up to 15°. The borders in some cases display traces of resorption. The felspar (plagioclase) is in the form usually of broad regular crystals, 3 to 4 mm. in size, and giving extinctions of andesine-labradorite (28°); they are “clouded” through the presence of fine alteration products associated with numerous fissures. The relation between the hornblende and the plagioclase is not constant. This appears to be partially due to the occurrence of traces of a groundmass.

The diorite forming blocks in the agglomerate of the coast cliffs, west of the Salt Lake Passage, is a remarkable rock showing large blackish hornblende crystals, in the shape of blades 25 mm. long and 3 or 4 mm. broad, set in a base of opaque plagioclase felspar which surrounds the hornblende. The last-named is deep-brown, very pleochroic, yields extinctions up to 22°, and displays but little evidence of resorption. The plagioclase is irregular in shape and exhibits broad lamellæ giving extinctions of acid labradorite (28°-30°). It is traversed by numerous fine fissures filled with decomposition products and contains abundant dust-like materials. (Spec. grav. 2·8).