342 A HISTORY OF FISHES 



up into a scries of divisions of various grades, corresponding to 

 the chapters, sections, and paragraphs of a book. The names 

 given to the different kinds of rock by the geologist, such as the 

 Chalk, Cambridge Greensand, Oolite, Red Sandstone, London 

 Clay, etc., may be ignored here, but the main divisions and 

 subdivisions into which geological time has been split up are of 

 greater importance, as it will be necessary to mention these by 

 name in the course of this chapter. The main divisions are 

 known as eras, each era being subdivided into several periods, 

 which may be further split up into epochs. Just as it is customary 

 to speak of ancient, mediaeval, and modern history of the 

 human race, so do geologists refer to the Palaeozoic, Mesozoic, 

 and Tertiary (or Caenozoic) eras in the history of the earth. 

 In the accompanying diagram (Fig. 122) the earliest era, the 

 Archaean or Pre-Cambrian, has been largely omitted, for 

 although it may represent more than the half of geological 

 time, and correspond to about thirty-two miles of thickness of 

 strata (all the remaining fossil-bearing rocks representing about 

 twenty-one miles), this time was passed before there were any 

 living organisms with structures sufficiently hard to form fossils. 

 The rest of the divisions from the Cambrian to the present day 

 are all included, and the sizes of the spaces in the diagram 

 represent very approximately the relative length of the periods 

 in geological time. Some idea of the time-scale may be gained 

 from the fact that it has been estimated that the lower layers 

 of the Triassic period, during which mammals first made their 

 appearance on earth, were deposited somewhere in the neigh- 

 bourhood of 175,000,000 years ago. Fishes first made their 

 appearance during the Silurian period, and became very 

 abundant during Devonian and Carboniferous times, when they 

 were the dominant type of animal life and had already produced 

 a large number of diverse types. 



It must not be supposed that the fossil-bearing strata always 

 have the regular arrangement depicted in the accompanying 

 diagram, or that the record of the rocks provides a continuous 

 story, in which the history of all the main groups of animals 

 and their lines of descent may be deciphered with the aid of 

 a complete series of well-preserved fossil remains. Quite often 

 the proper arrangement of the layers has been much disturbed, 

 the rocks being variously tilted, buckled, twisted, broken, or 

 even turned wrong way up, as a result of the shrinkage of the 

 earth's crust when cooHng down. The reading of the story of 

 the earth's history may be likened to the reading of a book, but a 



