Part 2

SILICATES.--These are by far the most abundantly occurring minerals. The species are also exceedingly numerous, but we may note here only a few of the more important. They are composed of silica and various bases, such as lime, potash, magnesia, soda, alumina, &c. _Augite_ or _pyroxene_ is a black or greenish-black mineral, found, either as crystals, which are generally small, or as rounded grains and angular fragments, in basaltic and volcanic rocks. It never occurs in granite rocks. It is brittle, and has a vitreous or resinous lustre. There are a number of varieties or sub-species of augite. _Hornblende_, like augite, also includes a great many minerals. When the crystals are small, it is often difficult to distinguish hornblende from augite. Common hornblende occurs crystallised or massive, and is dark green or black, with a vitreous lustre. It is generally sub-translucent. It usually crystallises in igneous rocks which contain much quartz or silica; while augite, on the other hand, crystallises in igneous rocks which are of a more basic character--that is to say, rocks in which silica is not so abundantly present. _Felspar_ is a generic term which embraces a number of species, such as _orthoclase_ or _potash-felspar_, _albite_ or _soda-felspar_, and _anorthite_ or _lime-felspar_. _Orthoclase_ is white, red or pink, and gray. It is one of the ordinary constituents of granite, and enters into the composition of many rocks. _Albite_ is usually white. It often occurs as a constituent of granite, not unfrequently being associated in the same rock with pink felspar or orthoclase. In syenite and greenstone it occurs more commonly than orthoclase. _Anorthite_ occurs in white translucent or transparent crystals. It is not so common a constituent of rocks as either of the other felspars just referred to. _Mica_: this term includes several minerals, which all agree in being highly cleavable into thin elastic flakes or laminae, which have a glistening metallic lustre. Mica is one of the common constituents of granite. _Talc_ is a silvery white, grayish, pale or dark-green coloured mineral, with a pearly lustre. It splits readily into thin flakes, which are flexible, but not elastic, and may be readily scratched with the nail. It is unctuous and greasy to the touch. It occurs in beds (_talc-slate_), and is often met with in districts occupied by metamorphic crystalline rocks. _Serpentine_ is generally of a green colour, but brown, red, and variously mottled varieties occur. It has a dull lustre, and is soft, and easily cut; it is tough, however, and takes on a good polish. It forms rock-masses in some places. The finer varieties are called _noble serpentine_. _Chlorite_ is another soft, easily scratched mineral, generally of a dark-green colour. It has a pearly lustre. Sometimes it occurs in beds (_chlorite-slate_), and is often found coating the walls of fissures in certain rocks. It has a somewhat greasy feel. The three last-mentioned minerals--talc, serpentine, and chlorite--are all silicates of magnesia. _Zeolites_ is a term which comprises a number of minerals of varying chemical composition, all of which tend to form a jelly when treated with acids. When heated by the blow-pipe they bubble up, owing to the escape of water; hence their name _zeolites_, from _zeo_, I boil, and _lithos_, a stone. The zeolites occur very commonly in cavities in igneous rocks, and also in mineral veins.

Having now mentioned the chief rock-forming minerals, we proceed to a brief description of some of the more typical representatives of the five great classes of rocks referred to at page 8.

PETROLOGY.[C]

[C] _Petros_, a rock, and _logos_, a discourse. Some geologists restrict this term to the study of the _structure_ and _arrangement of rock-masses_, and apply the term _lithology_ (_lithos_, a stone, and _logos_, a discourse) to the study of the _mineralogical composition of rocks_.

MECHANICALLY FORMED ROCKS.

17. (_A._) SEDIMENTARY CLASS.--Three of the most commonly occurring rocks of this class have already been described, but a few details are added here.

_Conglomerate._--This is a consolidated mass of more or less water-worn and rounded stones. These stones may be of any size. When they are very large, the rock is called a _coarse conglomerate_; the finer varieties, in which the stones are small, are known as _pebbly conglomerates_. The ingredients of a conglomerate may consist of any kind of rock, or of a mixture of many different kinds. When they consist entirely of quartz, the rock becomes _quartzose_. The finer-grained conglomerates usually shew lines of deposition or bedding, but in some of the coarser sorts it is often difficult to detect any kind of arrangement. The stones are usually imbedded in a matrix of quartzose grit and sand, but sometimes this is very scanty. When the nature of the material which binds the stones together is very well marked, the rock becomes _ferruginous_, _calcareous_, _arenaceous_, or _argillaceous_, according as the binding or cementing material is _iron_, _lime_, _sand_, or _clay_. _Breccia_ is a rock in which the included fragments are _angular_.

18. _Sandstone_ is, as already remarked, merely consolidated sand. The coarser varieties, in which the grains are as large and larger than turnip-seeds, are termed _grit_. From these coarse varieties the rock passes insensibly, in one direction, into a fine or pebbly conglomerate, and in another into a rock, so fine-grained that a lens is needed to distinguish the component particles. Quartz is the prevailing ingredient--sometimes clear, at other times white. Frequently, however, the grains are coated with an oxide of iron, which gives the resulting rock a red colour. The other colours assumed by sandstone--such as yellow, brown, green, &c.--are also in like manner due to the presence of some compound of iron. When mica or felspar occurs plentifully, we have, in the one case, _micaceous sandstone_, and in the other _felspathic sandstone_. A sandstone in which the grains are cemented by carbonate of lime is said to be _calcareous_. _Freestone_ is a sandstone which can be worked freely in any direction. In most sandstones, the lines of bedding are distinct; when they are so numerous as to render the rock fissile, the sandstone is said to be _shaly_.

_Shale_ is a more or less indurated fissile or laminated clay. When the rock becomes coarse by the admixture of sand, it gradually passes into a _shaly sandstone_. There are many other varieties of clay-rocks--such as _fire-clay_, _pipe-clay_, _marl_, _loam_, &c.--which are sufficiently familiar.

19. (_B._) EOLIAN or AERIAL CLASS.--_Blown-sand_ is found at many places on sea-coasts. It generally forms smooth rounded hummocks, which are sometimes arranged in long lines parallel to the trend of the coast, as, for example, in the Tents Moor, near St Andrews. The _sand-hills_ of deserts also belong to this class.

_Debris_ is the loose angular rubbish which collects at the base of cliffs, on hill-tops, and hill-slopes. Immense accumulations of it occur in lofty mountainous districts and in arctic regions. In Nova Zembla, for example, the solid rock of the country is almost concealed beneath a thick covering of debris. But the various kinds of debris will be more particularly described further on.

_Soil._--An account of this can hardly be given without entering into the theory of its origin, and therefore we reserve its consideration for the present.

CHEMICALLY FORMED ROCKS.

20. _Stalactites_ and _stalagmites_ are carbonates of lime. They vary in colour, being white, or yellow, or brown. Stalactites are usually found adhering to the roofs of limestone caverns, &c., or depending from limestone rocks; stalagmites, on the other hand, commonly occur on the floors of limestone caverns, where they often attain a thickness of many feet.

_Siliceous sinter_ is silica with the addition of water--in other words, a hydrated quartz. It is not a very abundant rock, and is found chiefly in volcanic countries.

_Rock-salt_ has already been described. It occurs either as thin beds, or in the form of thick cake-like masses, often reaching ninety or one hundred feet in thickness. It is rudely crystalline in texture, and is usually discoloured brown and red with various impurities.

ORGANICALLY DERIVED ROCKS.

21. _Limestone_ consists of carbonate of lime, but usually contains some impurities. The varieties of this rock are numerous; some of them are as follows: _Chalk_; _oolite_, a rock built up of little spheroidal concretions, whence its name, _egg_ or _roe stone_ (the coarser oolites are called _pisolite_, or _pea-stone_); _lacustrine limestone_, &c. When much silica is diffused through the rock, we have a _siliceous limestone_; the presence of clay and of carbonaceous matter gives us _argillaceous_ and _carbonaceous limestones_. _Cornstone_ is a limestone containing a large quantity of arenaceous matter or sand. Many limestones are distinguished by the different kinds of organic remains which they yield. Thus, we have _muschelkalk_ or _shell-limestone_, _nummulitic_, _crinoidal_, &c. limestone. The crystalline limestones, such as _statuary marble_, are metamorphosed limestones. Not a few limestones are chemically formed rocks, and many, also, are partly of chemical and partly of organic origin, so that no hard and fast line can be drawn between these two classes of rock.

_Dolomite_, or _magnesian limestone_.--This is a compound of carbonate of lime and carbonate of magnesia. Its colour is usually yellow, or yellowish brown, but gray and black varieties are sometimes met with. It is generally fine-grained, with a crystalline texture, and pearly lustre. It effervesces less freely with acids than pure limestone. In many cases dolomite is merely a metamorphosed limestone.

22. _Coal_ is composed of vegetable matter, but usually contains a greater or less percentage of impurities. The varieties of this substance are very numerous, and differ from each other principally in regard to their bituminous or non-bituminous character. Coal is bituminous or non-bituminous according as it is less or more highly mineralised. Bitumen results from the decomposition of vegetable matter; but, when the mineralising process (to which the formation of coal is due) has proceeded far enough, the vegetable matter gradually loses its bituminous character, and the result is a non-bituminous coal. Varieties of coal are the following: _Lignite_ or _brown coal_; _caking coal_; _cannel_, _parrot_, or _gas coal_; _splint coal_; _cherry_ or _soft coal_; _anthracite_ or _blind coal_, so called because it burns with no flame. _Peat_ may be mentioned as another natural fuel. It is composed of vegetable matter. In some kinds it is so far decomposed, or mineralised, that the eye does not detect vegetable fibres; when thoroughly dried, such peat breaks like a good lignite, and forms an excellent fuel.

METAMORPHIC ROCKS.

23. _Quartz-rock_, or _quartzite_, is an altered quartzose sandstone or grit; it is generally a white or grayish-yellow rock, very hard and compact. The original gritty character of the rock is distinct, but the granules appear as if they had been fused so far as to become mutually adherent. When the altered sandstone has been composed of grains of quartz, felspar, or mica, set in a siliceous, felspathic, or argillaceous base, we get a rock called _greywacke_, which is usually gray or grayish blue in colour.

24. _Clay-slate_ is a grayish blue, or green, fine-grained hard rock, which splits into numerous more or less thin laminae, which may or may not coincide with the original bedding. Most usually the 'cleavage,' as this fissile structure is termed, crosses the bedding at all angles.

25. _Crystalline limestone_ is an altered condition of common limestone. _Saccharoid marble_ is one of the fine varieties: it frequently contains flakes of mica. _Dolomite_, or magnesian limestone, already described, is probably in many cases an altered limestone; the carbonate of lime having been partially dissolved out and replaced by carbonate of magnesia. _Serpentine_ is also believed by some geologists to be a highly metamorphosed magnesian limestone.

26. _Schists_.--Under this term comes a great variety of crystalline rocks which all agree in having a foliated texture--that is to say, the constituent minerals are arranged in layers which usually, but not invariably, coincide with the original bedding. Amongst the schists come _mica-schist_ (quartz and mica in alternate layers); _chlorite-schist_ (chlorite with a little quartz, and sometimes with felspar or mica); _talc-schist_ (talc with quartz or felspar); _hornblende schist_ (hornblende with a variable quantity of felspar, and sometimes a little quartz); _gneiss_ (quartz, felspar, and mica).

27. _General Character of Metamorphic Rocks._--All these rocks betray their aqueous origin by the presence of more or less distinct lines of bedding. They consist of various kinds of arenaceous and argillaceous deposits, which, under the influence of certain metamorphic actions, to be described in the sequel, have lost their original granular texture, and become more or less distinctly crystallised. And not only so, but their chemical ingredients have in many cases entered into new relations, so as to give rise to minerals which existed either sparingly or not at all in the original rocks. Frequently, it is quite impossible to say what was the original condition of some metamorphic rocks; often, however, this is sufficiently obvious. Thus, highly micaceous sandstones, as they are traced into a metamorphic region, are seen to pass gradually into mica-schist. When the bedding of gneiss becomes entirely obliterated, it is often difficult to distinguish that rock from granite, and in many cases it appears to pass into a true granite.

28. _Granite_ is a crystalline compound of quartz, felspar (usually potash-felspar), and mica. Some geologists consider it to be invariably an igneous rock; but, as just stated, it sometimes passes into gneiss in such a way as to lead us to infer its metamorphic origin. There are certain areas of sandstone in the south of Scotland which are partially metamorphosed, and in these we may trace a gradual passage from highly baked felspathic sandstones with a sub-crystalline texture into a more crystalline rock which in places graduates into true granite. Granite, however, also occurs as an igneous rock.

29. _Syenite_ is a crystalline compound of a potash-felspar and hornblende, and quartz is frequently present. _Diorite_ is a crystalline aggregate of a soda-felspar and hornblende. Both syenite and diorite also occur as igneous rocks.

There are a number of other metamorphic rocks, but those mentioned are the most commonly occurring species.

IGNEOUS ROCKS.

30. _Subdivisions._--In their chemical and mineralogical composition, igneous rocks offer great variety; but they all agree in having felspar for their base. They may be roughly divided into two classes, distinguished by the relative quantity of silica which they contain. Those in which the silica ranges from about 50 to 70 or 80 per cent. form what is termed the _acidic_ group; while those in which the percentage of silica is less constitute the _basic_ group of igneous rocks, so called because they contain a large proportion of the heavier bases, such as _magnesia_, _lime_, oxides of iron and manganese, &c. Igneous rocks vary in texture from homogeneous, compact, and finely crystalline masses up to coarsely crystalline aggregates, in which the crystals may be more than an inch in diameter. Sometimes they are dull and earthy in texture, at other times vesicular. When the vesicles are filled up with some mineral, the rock is said to be _amygdaloidal_, from the almond shape assumed by the kernels filling the cavities. When single crystals of any mineral are scattered through a rock, so as to be readily distinguished from the compact or crystalline base, the rock becomes _porphyritic_.

ACIDIC OR FELSPATHIC GROUP.

31. _Trachyte_ (_trachys_, rough) is a pale or dark-gray rock, harsh and rough to the touch, in which felspar is the predominant mineral. It is a common product of eruption in modern volcanoes.

_Clinkstone_ or _phonolite_ is a greenish-gray, compact, felspathic rock, somewhat slaty or schistose, and weathers with a white crust. It gives a clear metallic sound under the hammer. It is a rock not met with among the older formations of the earth's crust, being confined to Tertiary (see table, p. 85) or still more recent times.

_Obsidian_ or _volcanic glass_ is usually black, brown, or green, and usually resembles a coarse bottle-glass. When it becomes vesicular, it passes gradually into the highly porous rock called _pumice_. It is eminently a geologically modern volcanic rock.

_Felstone_ is a reddish-gray, bluish, greenish, or yellowish, hard, compact, flinty-looking rock, composed of potash-felspar and silica. It is generally splintery under the hammer. Some varieties are slaty, and are frequently mistaken for clinkstone, which they closely resemble. When the quartz in felstone is distinctly visible either as grains or crystals, the rock passes into a _quartz-porphyry_.

_Granite_ is recognised as an igneous as well as a metamorphic rock. Sometimes the veins and dykes which proceed from or occur near a mass of granite contain no mica--this kind of rock is called _elvan_ or _elvanite_.

_Porphyrite_ or _felspathite_ includes a number of rocks which have a felspathic base, through which felspar crystals are scattered more or less abundantly. Sometimes hornblende, or augite, or mica is present. The colour is usually dark--some shade of blue, green, red, puce, purple, or brown--and the texture varies from compact and finely crystalline up to coarsely crystalline. Porphyrites are usually porphyritic, and frequently amygdaloidal.

AUGITIC AND HORNBLENDIC OR BASIC GROUP.

32. _Basalt_ is a dark or almost black compact homogeneous rock, composed of felspar and augite with magnetic iron. An olive-green mineral called _olivine_ is very frequently present. The coarser-grained basalts are called _dolerite_. The columnar structure is not peculiarly characteristic of basalt. Many basalts are not columnar, and not a few columnar rocks are not basalts.

_Greenstone_ or _diorite_ is usually a dull greenish rock, sometimes gray, however, speckled with green. It is composed of soda-felspar and hornblende. The fine-grained compact greenstones are called _aphanite_.

_Syenite_, like granite, is recognised as an igneous as well as a metamorphic rock. There are several other rocks which come into the basic group, but those mentioned are the more common and typical species.

33. _Fragmental Igneous Rocks._--All the igneous rocks briefly described above are more or less distinctly crystalline in texture. There is a class of igneous rocks, however, which do not present this character, but when fine-grained are dull and earthy in texture, and frequently consist merely of a rude agglomeration of rough angular fragments of various rocks. These form the FRAGMENTAL group of igneous rocks. The ejectamenta of loose materials which are thrown out during a volcanic eruption, consist in chief measure of fragments of lava, &c. of all sizes, from mere dust, sand, and grit, up to blocks of more than a ton in weight. These materials, as we shall afterwards see, are scattered round the orifice of eruption in more or less irregular beds. The terms applied to the varieties of ejectamenta found among modern volcanic accumulations, will be given and explained when we come to consider the nature of geological agencies. In the British Islands, and many other non-volcanic regions, we find besides crystalline igneous rocks, abundant traces of loose ejectamenta, which clearly prove the former presence of volcanoes. These materials are sometimes quite amorphous--that is to say, they shew no trace of water action--they have not been spread out in layers, but consist of rude tumultuous accumulations of angular and subangular fragments of igneous rocks. Such masses are termed _trappean agglomerate_ and _trappean breccia_. At other times, however, the ejectamenta give evidence of having been arranged by the action of water, the materials having been sifted and spread out in more or less regular layers. What were formerly rude breccias and agglomerates of angular stones now become _trappean conglomerates_--the stones having been rounded and water-worn--while the fine ingredients, the grit, and sand, and mud, form the rock called _trap tuff_. Fragmental rocks are often quite indurated--the matrix being as hard as the included stones. But as a rule they are less hard than crystalline igneous rocks, and in many cases are loose and crumbling. When a fragmental rock is composed chiefly of rocks belonging to the acidic group, we say it is _felspathic_. When augitic and hornblendic materials predominate, then other terms are used; as, for example, _dolerite tuff_, _greenstone tuff_.

STRUCTURE AND ARRANGEMENT OF ROCK-MASSES.

34. The student can hardly learn much about the mineralogical composition of rocks, without at the same time acquiring some knowledge of the manner of their occurrence in nature. We have already briefly described certain sedimentary rocks, such as conglomerate, sandstone, and shale, and have in some measure touched upon their structure as rock-masses. These rocks, as we have seen, are arranged in more or less thick layers or _beds_, which are piled one on the top of the other. Rocks which are so arranged are said to be _stratified_, and are termed _strata_. We may also use the word _stratum_ as an occasional substitute for _bed_. The planes of _bedding_ or _stratification_ are sometimes very close together, in other cases they are wide apart. When the separate beds are very thin, as in the case of shale, it is most usual to term them _laminae_, and to speak of the _lamination_ of a shale, as distinguished from the _bedding_ of a sandstone. Planes of bedding are generally more strongly marked than planes of lamination. The laminae frequently cohere, while beds seldom do. In the above figure, which represents a vertical cutting or _section_ through horizontal strata, the planes of lamination are shewn at _l, l, l_, and those of stratification at _s, s, s_. There are hardly any limits to the thickness of a bed--it may range from an inch up to many feet or yards, while _laminae_ vary in thickness from an inch downwards.

35. Hitherto we have been considering the _laminae_ and _strata_ as lying in an approximately horizontal plane. Sometimes, however, the layers of deposition in a single stratum are inclined at various angles to themselves, as in the following figure. This structure is called _false bedding_; the layers or laminae not coinciding with the planes of stratification. It owes its origin to shifting currents, such as the ebb and flow of the tide, and very often characterises deposits which have been formed in shallow water. (Hillocks of drifting sand frequently shew a similar structure, but their false bedding is, as a rule, much more pronounced.)