42 



seasons, and have failed to add a single specimen of either to their col- 

 lections. Their presence on Angel Island, which is situated in the gulf 

 about midway its length, may be readily explained when we consider 

 the geological changes that have taken place since the post-tertiary 

 period. The study of the geological formation of all the outlying islands 

 of the peninsula, both in the gulf and on the Pacific coast, convinced mo 

 that they were at one time portions of the mainland; the study of their 

 fauna strengthens this conclusion. ' Fringing the mainland, and over- 

 lying the other formations on the islands, is a deposit of post-tertiary 

 origin. There was an extensive upheaval during that period, and then 

 it was that the Islands probably formed a part of the peninsula, sub- 

 sidence taking place subsequently. So far has this depression gone 

 on — and it is probably going on at present — that some of the islands 

 are now separated from the mainland by water a thousand feet deep. 

 If we carry ourselves back to the time when they formed a part of the 

 mainland, we will have the mouth of the Colorado River discharging 

 its water into the gulf somewhere about the present site of Angel Island. 

 So that it is not difficult to account for the presence of these reptiles on 

 this now isolated bit of land, which was then connected with the Ari- 

 zonian region by a continuity of surface. 



