The Diplodocus 3 



perpendicular depth of fifteen miles or more ; not in one 

 place, but when they are arranged in chronological 

 order according to the times of their deposition. Their 

 strata have been studied carefully, have been classified, 

 and many of them named. 



Lying on the crystalline rocks, which do not appear 

 to have been laid down in water, there are great series 

 of strata, consisting at first of the debris of eroded 

 igneous rocks, which are known as Archean, in which 

 the evidence of the existence of life is mostly inferential, 

 based upon the fact that graphite and limestone occur 

 in these beds. Upon these were subsequently deposited 

 layers of mud, which settled down, when the world 

 was young, at the bottom of ancient seas and oceans. 

 In these are found here and there the remains of marine 

 animals and plants, mostly of a lowly organization. 

 To these very old strata geologists have applied the 

 name Paleozoic, because they contain relics of the most 

 ancient forms of life, the word ''paleozoic' being com- 

 posed of two Greek words which mean ; ' ancient" and 

 ; 'life." Superimposed upon these older formations 

 is another great series of rocky layers, some of which 

 were laid down in the seas, others of which were formed 

 on low-lying swampy lands, and still others in the beds 

 of rivers and estuaries. These strata geologists are 

 accustomed to call Mesozoic, the word being again 

 formed from two Greek vocables meaning "middle' 

 and "life." Still higher up in the ascending series is a 

 third great aggregation of stony beds, to which geolo- 

 gists have given the name Cenozoic, compounded of 

 Greek words which signify "new' and "life." These 

 beds are often called Tertiary. 



In the Paleozoic sandstones and limestones, as has 

 been intimated, we have the remains of creatures which 



