12 INTRODUCTION 



to the chapter on palaeontology, the science of prehistoric organ- 

 isms. The solid crust of the earth consists, geologically speaking, 

 of rocks, whether in the form in which we commonly conceive of 

 rocks, or in the form of gravel, sand, clay, or mud. These rocks 

 are, broadly speaking, of two kinds : the igneous rocks, which 

 are formed for the most part below the surface of the ground 

 and owe their structure to the action of heat, — they contain no 

 remains of organisms ; and the sedimentary or stratified rocks, 

 which have been formed for the most part under water by the 

 deposition of their constituents in layers or strata. In these 

 strata are found the remains of plants and animals; these 

 remains are known as fossils. 



A fossil, literally anything dug up, consists of the remains of 

 any organism which has been covered up by natural processes. 

 As is well known, the soft parts of organisms usually disintegrate 

 when the organism dies, so that, as a rule, only the hard parts 

 remain; thus we have the most abundant fossils from animals 

 which had hard parts, such as coral-skeletons, shells, and bones. 

 These hard parts may be preserved in their original condition 

 by the simple hardening into stone of the substance in which 

 they are imbedded ; or the original skeleton may be dissolved 

 away and replaced by a mineral deposit, frequently by silica, the 

 process being so gradual as to reproduce in great detail the 

 structure of the original, as we see in petrified trees ; in still 

 other cases fossils consist merely of impressions made by organ- 

 isms or parts of organisms in soft mud or clay, which has after- 

 ward become hardened, as in the fossil footprints of birds and 

 reptiles. 



The fossiliferous rocks were originally laid down in approxi- 

 mately horizontal, parallel strata, the oldest of course being the 

 deepest or farthest from the surface of the earth ; but in the 

 course of the earth's history these strata have become folded, 

 twisted, and broken in various ways, so that we find outcroppings 

 at the surface of the earth of strata of very different ages. The 

 classification of the various strata is pretty extensive, as they are 

 very numerous ; for our purpose it is sufficient to note the names 

 and order of succession of the main divisions, so that the student 

 may have an approximate idea of the relative position in the 

 series of any particular system of strata. There are four great 



