Part 8

92. _Proofs of Cosmical Changes of Climate._--From the preceding remarks it will be also apparent that fossils teach us much regarding the climatology of past ages. They tell us how the area of the British Islands has experienced many vicissitudes of climate, sometimes rejoicing in a warm or almost tropical temperature, at other times visited with a climate as severe as is now experienced in arctic and antarctic regions. Not only so, but we learn from fossils that Greenland once supported myrtles and other plants which are now only found growing under mild and genial climatic conditions; while, on the other hand, remains of arctic mammals are met with in the south of France. Such great changes of climate are due, according to Mr Croll, to variations in the eccentricity of the earth's orbit combined with the precession of the equinox. It is well known that the orbit of our earth becomes much more elliptical at certain irregularly recurring periods than it is at present. During a period of extreme ellipticity, the earth is, of course, much further away from the sun in _aphelion_[I] than it is at a time of moderate ellipticity, while, in _perihelion_,[J] it is considerably nearer. Now, let us suppose that, at a time when the ellipticity is great, the movement known as the precession of the equinox has changed the incidence of our seasons, so that our summer happens in perihelion and not in aphelion, while that of the southern hemisphere occurs in aphelion, and not, as at present, in perihelion. Under such conditions, the climate of the globe would experience a complete change. In the northern hemisphere, so long and intensely cold would the winter be, that all the moisture that fell would fall as rain, and although the summer would be very warm, it would nevertheless be very short, and the heat then received would be insufficient to melt the snow and ice which had accumulated during the winter. Thus gradually snow and ice would cover all the lands down to temperate latitudes. In the southern hemisphere, the reverse of all this would obtain. The winter there would be short and mild, and the summer, although cool, would be very long. But such changes would bring into action a whole series of physical agencies, every one of which would tend still further to increase the difference between the climates of the two hemispheres. Owing to the vast accumulation of snow and ice in the northern hemisphere, the difference of temperature between equatorial and temperate and polar regions would be greater in that hemisphere than in the southern. Hence the winds blowing from the north would be more powerful than those coming from the southern and warmer hemisphere, and consequently the warm water of the tropics would necessarily be impelled into the southern ocean. This would tend still further to lower the temperature of our hemisphere, while, at the same time, it would raise correspondingly the temperature at our antipodes. The general result would be, that in our hemisphere ice and snow would cover the ground down to low temperate latitudes--the British Islands being completely smothered under a great sea of confluent glaciers. In the southern hemisphere, on the contrary, a kind of perennial summer would reign even up to the pole. Such conditions would last for some ten or twelve thousand years, and then, owing to the precession of the equinox, a complete change would come about--the ice-cap would disappear from the north, and be replaced by continuous summer, while at the same time an excessively severe or glacial climate would characterise the south; and such great changes would occur several times during each prolonged epoch of great eccentricity. This, in few words, is an outline of Mr Croll's theory. That theory is at present _sub judice_, but there can be no doubt that it gives a reasonable explanation of many geological facts which have hitherto been inexplicable. Of course, it is not maintained that all changes of climate are due directly or indirectly to astronomical causes. Local changes of climate--changes affecting limited regions--may be induced by mutations of land and sea, resulting in the partial deflection of ocean currents, which are the chief secondary means employed by nature for the distribution of heat over the globe's surface.

[I] _Apo_, away from; _helios_, the sun.

[J] _Peri_, round about or near by; _helios_, the sun.

From what has been stated in the foregoing paragraphs, it is clear that in our endeavours to decipher the geological history of our planet, palaeontological must go hand in hand with stratigraphical evidence. We may indeed learn much from the mode of arrangement of the rocks themselves. But the test of superposition does not always avail us. It is often hard, and sometimes quite impossible, to tell from stratigraphical evidence which are the older rocks of a district. In the absence of fossils we must frequently be in doubt. But physical evidence alone will often afford us much and varied information. It will shew us what was land and what sea at some former period; it will indicate to us the sites of ancient igneous action; it will tell us of rivers, and lakes, and seas which have long since passed away. Nay, in some cases, it will even convince us that certain great climatic changes have taken place, by pointing out to us the markings, and debris, and wandered blocks which are the sure traces of ice action, whether of glaciers or icebergs. The results obtained by combining physical and palaeontological evidence form what is termed Historical Geology.

HISTORICAL GEOLOGY.

93. The fossiliferous strata, as they are generally termed, have been chronologically arranged in a series of _formations_, each of which is characterised by its own peculiar suites of fossils. Their relative age has been determined, as we have indicated above, by their fossils, and also by certain physical tests, the chief of these being _superposition_. It holds invariably true that a formation, A, found resting upon another series of strata, B, will always occur in precisely the same position, wherever these two deposits occur together. If B should appear in some place as resting upon A, we may be sure that the beds have been inverted during the contortion of the strata consequent upon subterranean action (see fig. 11, page 34). Again, another useful test of the relative age of strata lies in the circumstance that one is often made up or contains fragments of the other. In this case, then, it is quite clear which is the more recent accumulation. These tests have now been applied to the strata in many parts of the world, and the result is that geologists have been able to arrive at a chronological arrangement or classification, and so to construct a table shewing the relative position which would be occupied by all the different formations, if these occurred together in one place. In the British Islands the long series of strata is well developed, but many of the formations are much more meagrely represented than their equivalents in other countries. But even when we attempt to fill up the blanks in our own series by dovetailing with them the strata of foreign countries, there yet remain numerous breaks in the succession, pointing to the fact that the stony record is a very fragmentary one at the best. No doubt there are many large tracts of the earth's surface which have not yet been investigated, and when these are known we may hope to have our knowledge greatly increased. But no one who reflects upon the mode of origin of the fossiliferous strata, and the wonderful mutations which the earth has undergone, can reasonably anticipate that a perfect and complete record of the geological history of our planet shall ever be compiled from the broken and fragmentary testimony of the rocks.

94. The following table gives the names of the different formations arranged in the order of their superposition, the youngest being at the top, and the oldest known at the bottom:

IV. POST-TERTIARY OR QUATERNARY-- Historical or Recent. Pleistocene.

III. TERTIARY OR CAINOZOIC-- Pliocene. Miocene. Eocene.

II. SECONDARY OR MESOZOIC-- Cretaceous. Jurassic. Triassic.

I. PRIMARY OR PALAEOZOIC-- Permian. Carboniferous. Devonian and Old Red Sandstone. Silurian. Cambrian. Laurentian or Pre-Cambrian.

95. The PRIMARY formations are so called because they are the oldest known to us: they are not necessarily the first-formed aqueous deposits. Dr Hutton said truly: There is no trace of a beginning, and no signs of an end. In the PRIMARY or PALAEOZOIC (ancient-life) formations are found the earliest traces of life. The forms as a rule depart very widely from those with which we are acquainted now. The _Laurentian_ rocks have yielded only one fossil--a large foraminifer named _Eozoon Canadense_. The _Cambrian_ formation contains but few fossils--crustaceans, molluscs, zoophytes, and worm-tracks. The _Silurian_ strata are often abundantly fossiliferous. All the great classes of invertebrates are represented, and fish remains also occur. The _Devonian_ and _Old Red Sandstone_ are also characterised by the presence of an abundant fauna. In the Old Red Sandstone are numerous fish remains; it appears to have been an estuarine or lacustrine deposit; the Devonian, on the other hand, was marine, like the Silurian and Cambrian. The _Carboniferous_ formation is the chief repository of coal in Britain. It consists of terrestrial, fresh or brackish water, and marine deposits. The fauna and flora of the _Permian_, which is partly a marine and partly a fresh-water formation, are allied, upon the whole, to those of the Carboniferous, but offer at the same time many contrasts.

96. The SECONDARY OR MESOZOIC (middle-life) formations contain assemblages of fossils which do not depart so widely from analogous living forms as those belonging to Palaeozoic times. The _Triassic_ strata yield abundance of rock-salt. In Britain they contain very few fossils, but these are more abundant in the Triassic deposits of foreign countries. The oldest known mammals first appear in this formation. The _Jurassic_ formation is very highly fossiliferous. It is distinguished by the occurrence of numerous reptilian remains. Nearly all the beds of this formation are marine, but there are associated with these the remains of a forest or old land surface, and a considerable accumulation of estuarine or fresh-water deposits; impure coals also occur in this formation. The _Cretaceous_ strata are almost wholly marine, and chiefly of deep-water origin. But some land-plants are found, chiefly ferns, conifers, and cycads. Near the base of the formation occurs a great river deposit (Weald clay) with numerous remains of reptiles.

97. Among the oldest strata of the TERTIARY or CAINOZOIC (recent-life) division we meet with the _dawn_ of the existing state of the testaceous fauna--the _Eocene_ (_eos_, dawn, and _kainos_, recent) containing three and a half per cent. of recent species among its shells. The proportion of recent species increases in the _Miocene_ (_meion_, less, and _kainos_, recent), although the majority of the molluscs entombed in that formation belong to extinct species. In the _Pliocene_ (_pleion_, more, and _kainos_, recent), however, the extinct species are in a minority.

The POST-TERTIARY or QUATERNARY division comprises the concluding chapters of geological history. The _Pleistocene_ (_pleistos_, most, and _kainos_, recent) contains no extinct species of shells, but a number of extinct mammalia. In the _Recent_ deposits all the species of animals and plants are living. The Tertiary and Quaternary formations are partly of marine and partly of terrestrial and fresh-water origin. At the close of the Tertiary period the 'glacial epoch' of Pleistocene times began, and the British Islands and a large part of northern Europe and North America were then cased in snow and ice. Traces of glacial conditions have also been met with in the Eocene and Miocene. The evidence furnished by Palaeozoic and Mesozoic formations points chiefly to mild, genial, and sometimes tropical conditions. But traces of ice action are occasionally noted (namely, in the Silurian, Old Red Sandstone, Carboniferous, Permian, and Cretaceous formations), pointing, perhaps, in some of the cases, to former alternations of cold and warm periods. Indeed, the belief is now gaining ground, that the so-called glacial epoch of Pleistocene times was not one long continuous age of ice, but rather consisted of an alternation of warm and cold periods. And it is not improbable, but highly likely, that similar alternations of climate have happened during every period of great eccentricity of the earth's orbit.

QUESTIONS.

Section 1. What is Geology?

2. Define the term _rock_. How many classes of rock are there?

3, 4, 5. Into what groups are the mechanically formed rocks divided? Define the terms _conglomerate_, _sandstone_, and _shale_.

6. What is the nature of the rocks belonging to the Aerial or Eolian group?

7. Give an example of a chemically formed rock.

8. Give examples of organically derived rocks.

9. What kinds of rocks are embraced by the Metamorphic class?

10. What are igneous rocks?

12. What is the mineralogical composition of granite?

13. What is meant by a _mineral_?

14. Name five minerals which do not contain oxygen. Where does _fluor-spar_ occur? What is the element that enters most largely into the composition of the earth's crust?

15. Name the forms under which the mineral _quartz_ occurs. Name some of the oxides of iron. What is _iron pyrites_?

16. Name two _sulphates_. Name two _carbonates_. Name some of the _silicates_. In what kinds of rock is _augite_ found? Where does it never occur? In what kinds of rock does _hornblende_ usually occur? Mention three species of felspar. What is one of the most distinguishing characteristics of mica? Name three silicates of magnesia. Mention some of their distinguishing peculiarities. Where do _zeolites_ commonly occur?

17. What is a _quartzose conglomerate_? What is a _calcareous conglomerate_?

18. What is _grit_? What is _freestone_? To what are the various colours of sandstone due? What is _shale_?

19. Name some typical Eolian rocks, and tell where they occur.

20. How do _stalactites_ and _stalagmites_ occur? What is _siliceous sinter_, and how does it occur? How does _rock-salt_ occur?

21. Mention some of the varieties of limestone. What is _cornstone_? What is the composition of _dolomite_?

22. Name some of the varieties of coal.

23. What is _quartzite_?

24. Describe _clay-slate_.

25. Mention some altered limestones.

26. What are _schists_? Name and give the mineralogical composition of three schists.

27. What is the general character of metamorphic rocks?

28. How would you classify granite?

29. What is the mineralogical composition of _syenite_ and _diorite_?

30. How do we distinguish the two groups into which igneous rocks are subdivided? What is meant by the terms _amygdaloidal_ and _porphyritic_?

31. Name some rocks that belong to the acidic group. What is _quartz-porphyry_?

32. Give examples of augitic igneous rocks. Name a hornblendic igneous rock.

33. What are fragmental igneous rocks? What is the difference between _trappean breccia_ and _trappean conglomerate_?

34. What is meant by the terms _stratum_, _strata_, and _stratified_? What is the difference between _lamination_ and _bedding_? What is a section?

35. What is _false bedding_?

36. Briefly describe the general appearance of _mud-cracks_ and _rain-prints_, and say how these have been formed.

37. What is meant by a _succession of strata_?

38. Which kinds of stratified rocks generally have the greatest extension?

39. How do beds terminate?

40. How may planes of bedding sometimes indicate a break in the succession of strata?

41. What is the nature of _joints_? What are _master-joints_, and what is their probable cause?

42. What is _cleavage_, and what is its effect upon the bedding of rocks?

43. What is _foliation_?

44. Give examples of concretionary rocks. What is the nature of chert and flint nodules?

45. Define the terms _dip_ and _strike_. What is the _crop_ of a bed? What are _anticlines_ and _synclines_?

46. What is meant by an _inversion of strata_?

47. How does contemporaneous erosion indicate a pause in the deposition of a series of strata?

48. What is meant by _unconformability_? How does unconformability prove a lapse of time between the accumulation of the underlying and overlying strata?

49. What is _overlap_?

50. What is a _fault_? What is _hade_? How are the strata affected on either side of a fault? What is the appearance called _slickensides_? Under what circumstances should we term a fault a _downthrow_? and when should we term it an _upcast_? How is the approximate age of a fault sometimes shewn?

51. What are metamorphic rocks, and what is their general appearance? In what districts of the British Islands are they most abundantly developed? What are some of the appearances relied upon for distinguishing metamorphic from igneous granite?

52. How do igneous rocks occur? Define what is meant by _contemporaneous_ and _subsequent_ or _intrusive_ igneous rocks. How does a contemporaneous igneous rock affect the beds upon which it rests? What is the character of the bed overlying a contemporaneous rock? What is the general structure of a contemporaneous igneous rock? What is meant by _vesicular structure_? What is the general texture of a contemporaneous igneous rock? What is the nature of the jointing in igneous rocks? What is _wacke_?

53. What is the nature of the beds of _breccia_, _conglomerate_, _ash_, and _tuff_, with which contemporaneous igneous rocks are often associated? What is a _neck_ of _volcanic agglomerate_? How are the strata affected at their junction with a 'neck'?

54. How do intrusive igneous rocks occur? How do intrusive _sheets_ occur? What effect have they produced upon the strata above and below them? What is a _dyke_? What relation do they occasionally bear to _sheets_ of igneous rock? What is a _neck_ of intrusive igneous rock, and how have the strata surrounding it been affected?

55. Mention some of the contrasts between _intrusive_ and contemporaneous igneous rocks. What alteration is produced upon coal with which an intrusive sheet has come in contact?

56. What are _mineral veins_? What is the nature of the quartz veins in granite? How are the minerals usually arranged in the great metalliferous veins? What is a _pipe-vein_?

57. What are the great geological agents of change?

58. What is meant by _weathering_? How are rocks affected at the surface in tropical countries? What chemical effect has the atmosphere on calcareous rocks? How is soil formed? How are sand dunes formed? Mention some effects of the transporting power of the atmosphere.

59. Mention some of the chemical effects of interstitial water. What is the origin of _travertine_ or _calcareous tufa_?

60. How have _stalactites_ and _stalagmites_ been formed? Give some instances of the solvent power of springs.

61. How are caves in limestone formed? Describe some of the appearances of a country composed of calcareous rocks. Describe briefly how a river erodes its channel.

62. Describe the geological action of rain.

63. What do chemical analyses of river-water prove? Give an example. What are pot-holes? Give an example of the erosive power of running water. What amount of mud is carried in suspension by the Mississippi, and discharged annually into the sea? What estimate has been formed of the total amount of mineral matter annually transported by that river?

64. What is _alluvium_? How is it formed? and mention some examples of its occurrence.

65. How is sediment deposited by a river in a lake?

66. What is the difference between lacustrine and fluvio-marine deposits? What is a _delta_?

67. Describe the geological action of frost. Describe the geological action of river-ice.

68. What are _glaciers_? What thickness do they attain in the Alps? What is their rate of motion? What are _crevasses_, and how do they originate? What are _superficial moraines_? What are _terminal moraines_? What changes does a glacier effect upon its bed, and how are these modifications produced? What is the character of a glacial river? What is the origin of _icebergs_? How is the general absence of blocks and stones in Greenland icebergs to be explained? What is the nature of a submarine terminal moraine? What is the _ice-foot_? What is the chief agent in distributing erratic stones and blocks over the sea-bottom? What effect upon the sea-bed must stranding icebergs produce?

69. What are some of the chemical compounds held in solution in sea-water? Which of these go to form the shells and skeletons of marine animals?

70. Describe the action of breakers on a sea-coast. How does frost aid the wasting action of breakers? What effect has the nature of the rocks in the production of inequalities in a coast-line? Upon what part of the sea-bottom does the material derived by the action of the breakers chiefly accumulate? What effect have the tides and ocean currents in the distribution of sediment?

71. What is the general rule as regards fine-grained and coarse-grained deposits? Mention a partial exception to this rule. What effect have tidal currents in shallow seas?

72. How are rocks disintegrated through the action of plants? What is peat? What may be inferred from the occurrence of shell-marl underneath peat? What does the appearance of roots and trunks of trees, and of remains of land animals under peat, indicate?

73. What, generally, is the geological action of animal life?

74. What is coral? What is a _fringing_ reef? What is the general character of a _barrier_ reef? Give an example of one. What is an _atoll_? What is the nature of coral rock? What is Mr Darwin's theory of the formation of coral reefs?

75. What is the nature of the Atlantic ooze? In what respects may it eventually come to resemble chalk and limestone? Mention an instance of the abundant occurrence in the sea of animalcules with siliceous coverings and skeletons. What is the nature of the red clay found at great depths in the Atlantic and Southern Oceans?

76. What are some of the notions held in regard to the internal condition of the earth? At what (average) rate does the temperature of the earth's crust increase as we descend from the surface?

77. What is the nature of the movements to which the earth's crust is subjected?

78. Describe the hypotheses advanced to account for earthquakes. Mention some of the effects of earthquakes--1_st_, as regards alterations of level; and 2_d_, as regards modifications of the surface.

79. Mention a good example of tranquil elevation and depression of the earth's crust. Mention some of the proofs of an elevatory movement. Give proofs that shew depression of the land. How may certain former changes of sea-level be accounted for without inferring any movement of the land?

80. What effect must _depression_ have upon the strata forming the earth's crust? What is the result of a movement of elevation? What is the cause of _cleavage_?

81. What is the nature of the materials thrown out during volcanic eruptions? What is the general structure of a volcanic cone? How does molten rock make its escape from the orifice of eruption? What is the meaning of the terms _lapillo_, _puzzolana_, and _ceneri_?