TEANSITIOtf EOCKS. 59 



quite possible, will be presently shown in describing the 

 chalk formation. 



Ages of comparative quiet now appear to have succeeded 

 the first great contraction of the earth's crust, probably 

 millions of years, during which time the tides and currents 

 of the ocean had to wash and wear down all the thousands 

 of projecting rocks or inequalities and dissolve (as before 

 described) all the lime, depositing the sand and clay in those 

 immense strata which form the "transition series;" this 

 appears to have taken place over nearly the whole world at 

 that time, and ages upon ages must have elapsed to form 

 such deposits as the sandstone, claystone, and limestone, 

 in alternation, forming the " Llandilo," " Caradoc," and 

 " Wenlock " strata, more than a mile in thickness ; these 

 are by some geologists reckoned among the primary series 

 (by some called the "transition rocks"), and in England 

 form the " Cambrian " and " Silurian " systems. 



In these strata the remains of organised beings are first 

 found, consisting of zoophytes, Crustacea (chiefly Tri- 

 lobites, fig. 11), nautili, crinoidii (stone lilies), and a few 



"BIG. 11. TRILOBITE. 



ganoid (plate- covered) fishes ; these lower forms of animal 

 life in some parts abound in the most prodigious numbers. 

 There must, of course, have been vegetables of some kind 

 previously formed to constitute nourishment for these 

 animals, but scarcely any remains of such exist, except in a 

 few localities. 



The Trilobites were amongst the first creatures inhabiting 

 our globe, and it is a curious fact to contemplate, that their 

 eyes (fig. 12) should have been preserved perfect; they 



