114 



THE POPULAR EDUCATOR. 



We are more likely to have registered all the species of exist- 

 ing molluscs which inhabit our seas, than to have discovered 

 individuals of every species which lie entombed in the rocks of 

 our island ; and yet, as far as our research has gone, the living 

 species are not a tithe of the number of fossil Testacea already 

 known by us. 



This means to say that our rocks chronicle the history of the 

 rise and fall of ten complete populations of the seas which girt 

 our island. We have no reason to think that the land animals 

 change less slowly than the species which inhabit the sea, and 

 yet we only discover that fossil species of Mammalia are more 

 numerous by one-half than those which now exist ; because, as 

 we have said, when we do discover such a fossil, we have 

 happened to hit upon the place where a land animal was 

 accidentally deposited amid the accumulating sediment. 



TJie Distribution of Fossils. From the foregoing remarks the 

 reader will readily perceive that the distribution of fossils is not 

 indiscriminate, but is regulated by certain considerations. The 

 fossils contained in one stratum of rock represent that order of 

 life which was existing at the time the rock was being deposited ; 

 and if, at some distant place, the same kind of rock was 

 discovered in which we found one fossil similar to any of those 

 which the former rock contained, we should continue our search, 

 confidently expecting to find other members of the same group. 



For instance, the Mediterranean Sea may be looked upon as 

 one area of life. Occasionally we find a species peculiar to one 

 locality, but the great majority of shells is common to all the 

 coasts. The rivers which discharge themselves into that sea 

 are depositing strata. Suppose that the bed were elevated, and 

 the rocks found by the Bhone, the Po, and the Nile were 

 examined, the mineral matter of the rocks would vary, but the 

 fossils would be well nigh all the same. 



From this instance, which is in progress under our observa- 

 tion, we deduce another fact, that although the mineral matter 

 which composes rocks may be essentially different, yet, provided 

 that the deposition of those rocks be during the same period and 

 within the same area of life, their fossil contents will bear a 

 close resemblance ; and it is possible that every species in the 

 one will be found in the other. 



Yet it must not be concluded that rocks deposited at the same 

 time of necessity contain the same fossils. A deposition of 

 strata may be going on off the shores of Siberia, and another in 

 the Bay of Bengal ; but no one will expect that these two areas 

 of deposition are included in the same area of life. In the one 

 an Arctic fauna, or a generation of Arctic life, is being enf ossilled, 

 and in the other the fauna is truly tropical ; and yet, for all that, 

 the conditions of climate are so widely different, that certain 

 cosmopolite species will be detected in each, and, to use Von 

 Buch's phrase, a peculiar " facies," or general resemblance a 

 family likeness will pervade the character of the two groups ; 

 .so that, although two classes of rocks may have been deposited 

 at very distant places and under very different conditions, but 

 at the same period, the geologist is seldom at fault to show that 

 such is the case from common characteristics possessed by their 

 fossils, although the majority of those fossils belong to different 

 species. 



The distribution of life, and the laws which regulate it, is a 

 most interesting subject ; but, from our imperfect knowledge 

 of Palaeontology, it can only be studied with regard to living 

 species. From such study, however, we have learned sufficient to 

 teach us that it is very dangerous to assert what may have been 

 the climate and the nature of the country when a certain species, 

 now in a fossil condition, were in existence. Experience, how- 

 ever, seems to point out that all changes in species which 

 occupied a certain area were very gradual ; and if the fossil con- 

 tents of two adjacent strata are widely different from each other, 

 it argues that long ages interposed between their respective 

 deposition, and that a series of intermediate deposits are wanting, 

 which, had they been present, would have established a gradation 

 in the forms of life. 



CLASSIFICATION OF KOCKS. 



Three prominent characteristics aid us in classifying rocks 

 according to their chronological order : (1) Their order of 

 superposition ; (2) their mineral characters ; and (3) their fossil 

 contents. If two strata are discovered, one lying on the other, 

 we may at once infer that the one which is beneath was 

 deposited before the other. Cases, however, are known where, 



by a tremendous upheaval of the surface, a series of strata has 

 been actually turned over ; but this is very local, and not at all 

 difficult to discover. Yet the order of superposition is not always 

 satisfactory, for it very frequently happens that between the- 

 deposition of the two strata vast periods of time intervened ; 

 and, to complete the series, it would be necessary to insert 

 perhaps several strata between the two adjacent rocks strata 

 which are found to have been deposited in this very period in 

 other areas. For instance, suppose England were submerged, 

 and a layer of rock deposited over the whole surface of our 

 island, in Wales this newly-deposited rock would rest on the old 

 Silurians rocks hoary with the vastness of their age while 

 this same rock is found on the opposite coast to be resting on the 

 tertiary formations that is, those last upheaved from the ocean 

 bed. Obviously it would be a great error to conclude that, 

 because this new rock was found superimposed upon the Silurian, 

 therefore it was deposited immediately after that foundation. 



Hence, although the order of superposition may and does 

 determine the relative times of deposition of the whole series of 

 rocks, yet we cannot infer that any one rock was immediately 

 deposited after another because it happens to lie upon it. 



The mineral character of a rock, although it may be put in 

 as evidence as to its age, yet is by no means conclusive. For 

 we find the same kind of rock appearing again and again in the 

 different formations. This a glance at the following tabulations 

 will at once decide ; and, moreover, it is not unusual to find a 

 limestone growing gradually more arenaceous until it became a 

 sandstone, which merely means to say that the area of the 

 deposition of the limestone was on the same level and con- 

 tiguous to the area of the deposition of the sandstone ; but we 

 cannot assert that the two rocks could not be deposited at the 

 same time because of their different mineral characters. 



The organic remains in a rock are the surest of all tests of age. 

 The same fossils may be found in several succeeding strata, but 

 every strata has some fossils peculiar to itself by which it is at 

 once known, and referred to its proper position. A knowledge 

 of Palaeontology is therefore absolutely necessary to the miner. 

 Many a coal shaft has been sunk in a class of rocks which never 

 yield coal, and that, too, when every bucket brought up silent 

 witnesses, which, had they found an interpreter, would have pre- 

 vented much useless expenditure. 



As we describe the various formations, we shall note and 

 delineate those fossils which are characteristic of them. 



The earliest classification seems to have been made by Steno 

 in 1669, who divided all rocks into primary and secondary. 

 Some ten years later, Leibnitz improved upon this nomenclature 

 by substituting the words stratified and unstratified. 



Then came Werner and Hutton. The former conceived that 

 the deposition of all existing rocks was due to the action of 

 water ; whereas the Vulcanists, the followers of Hutton, differed 

 from the Neptunists in ascribing the production of the Primary 

 and Transition rocks to igneous action. Each of these leaders 

 classified the rocks according to their theories. 



At the beginning of this century, William Smith, " the father 

 of English geology," commenced his labours, and from him a 

 race of investigating geologists has sprung, by whom the rocks 

 have been classified in the following order. The formations are 

 in a descending order : 



1. Post-Tertiary. The latest ac- 



cumulations. 



2. Tertiary. 



3. Cretaceous or Chalk. 



4. Oolitic. 



5. Triassic. 



6. Permian.orNewRedSandstone. 



7. Carboniferous or Coal System. 



8. Devonian, or Old Bed Sand- 



stone. 



9. Silurian. 



10. Metamorphic. 



Below these lie the Primary Eocks, but it must not be sup- 

 posed that the rocks of igneous origin are not found in the 

 stratified rocks, for it will be shown that granite was in a 

 state of fluidity when the Oolite was being, deposited ; but we 

 shall explain this in our next lesson. 



A division of the rocks has been made according to their 

 fossils, or the types of life they exhibit, as compared with our 

 present orders of life : 



1. Cainozoic period ( Post-tertiary. 

 (Recent life) 



/ Permian. 



3. Paleozoic Pe-l Carboniferous . 



riod ( Ancient < Devonian . 

 life) . . . ( Silurian . 



4. Azoic period ) Metam0rpnic , 



(No life) . ,JL 



