Geology Part 5

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 circ.u.mstance 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 acc.u.mulation. 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 cla.s.sification, 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 antic.i.p.ate 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.

Jura.s.sic.

Tria.s.sic.

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 cla.s.ses 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 a.s.semblages of fossils which do not depart so widely from a.n.a.logous living forms as those belonging to Palaeozoic times. The _Tria.s.sic_ strata yield abundance of rock-salt. In Britain they contain very few fossils, but these are more abundant in the Tria.s.sic deposits of foreign countries. The oldest known mammals first appear in this formation. The _Jura.s.sic_ 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 a.s.sociated with these the remains of a forest or old land surface, and a considerable acc.u.mulation 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 sh.e.l.ls. 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 sh.e.l.ls, 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...o...b..t.

QUESTIONS.

Section 1. What is Geology?

2. Define the term _rock_. How many cla.s.ses 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 cla.s.s?

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 distinguis.h.i.+ng characteristics of mica? Name three silicates of magnesia. Mention some of their distinguis.h.i.+ng 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 _stalact.i.tes_ 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 cla.s.sify granite?