162 GEOLOGICAL SITUATION AND 



have nearly the same antiquity. (For a more particular account of the 

 strata in which these fossils are sometimes found, see Appendix I.) 



This fact — the occurrence of the bones of the elk and Mastodon in 



shell-marl — may seem to afford a satisfactory reason for their preservation. 



A little attention, however, will show that this opinion is not so well 



founded as might at first appear. 



Mari In many of the marl deposits, the layer is not sufficiently thick to 



Deposits. 



contain the whole of the Mastodon skeleton ; some parts lying above, and 

 some below, the marl. In the skeleton we have described, the vertex or 

 summit of the head was near the surface of the marl; and, although a 

 considerable part of the skeleton lay in this bed, yet, as this was only three 

 feet thick, it could not envelop an animal whose height, in the recent state, 

 was twelve feet or more ; and, of consequence, part of the bones would be 

 buried in the clay or mud, on which the marly layer was formed (vide 

 Yignette). Now, the Mastodon bones which lay under the marl are equally 

 well preserved with those in it. 



In relation to the strata which imbed the Mastodon relics in South 

 America, an important fact is mentioned by M. D'Orbigny. In his list of 

 formations, he makes the Pampeen, in which these remains are found, to 

 lie "between the tertiary and diluvial formations/' — a situation corre- 

 sponding with the geological position of the North American relics of the 

 same family. 

 causes of What, then, are the causes to which we must attribute the remarkable 



Preserva- 

 tion, preservation of a few skeletons, while thousands scattered on the earth's 



surface have disappeared ? The decomposition of the latter is attributable 



to the combined action of air and water, aided by the frosts of winter and 



Dry Air. the suns of summer. Perfectly dry air does not favor the decomposition 



