1 88 Marsh. 



what I told you, when we were on the hillside, about the 

 Gi-lobigerina ooze now being deposited at the bottom of the 

 Atlantic. The total thickness of these rocks has been esti- 

 mated to be more than twelve miles. During their deposi- 

 tion the remains of animals and plants have been buried in 

 them, and the hard and bony parts of these undergoing 

 petrification have thus been preserved as fossils. 



The stratified rocks have been arranged by geologists in 

 four groups. Of these, the oldest is termed the Primary, 

 the next oldest the Secondary, then the Tertiary, whilst 

 the most recent is called the Post-Tertiary or Quaternary. 

 Each of these groups is sub-divided into systems, and these 

 again into formations, each formation being marked by 

 characteristic fossils. In the Primary group the most 

 important systems are the Silurian, the Devonian and the 

 Carboniferous. The Secondary group is characterised by 

 the abundance of beds of limestone ; the Lias, the Oolite 

 and the Cretaceous are its chief systems. The Tertiary and 

 the Post-Tertiary groups consist more particularly of shallow 

 alluvial and estuarine deposits. 



As we study the various formations and the fossils con- 

 tained in them, we find that at different periods in the his- 

 tory of the earth most of the leading classes of animals and 

 plants attained a high state of development, and having 

 reached their zenith, gradually declined. 



We have already alluded to this point as illustrated by 

 the vegetable world ; here we will consider it in relation to 

 the animal creation. The Old Red Sandstone, which lies 

 below the Carboniferous rocks, is remarkable for the high 

 degree of development exhibited by its fossil fishes. When, 

 riter long ages, we come to the era of the Liassic and Oolitic 



