THE TERTIARY LACUSTRINE AREAS OF THE CORDILLERAS. 115 



Rocky Mountains and the Sierra Nevada were covered by fre.sli water, Ibrm- 

 in^- interior lakes, the exact dimensions of wliich have not yet been made 

 out, but ot which enough has become known to convey a vivid i(]ea of the 

 existence of a lacustrine development surpassing in magnitude anything now 

 existing on the earth's surface. For reasons which will be readily appre- 

 ciated, our studies of the relative areas covered by land and water at different 

 geological times must be limited to the occurrence of fresh water. We are 

 not quite advanced enough in this discussion to take uj) the question of the 

 changes of climate which have been continually going on during the geo- 

 logical times in consequence of the ever-increasing area of the land masses 

 and the diminution of the ocean sui'face. We know from the investigations 

 of the various surveys carried on in the Cordilleras, that the ocean has been 

 shut out from a large part of the region between the Eocky Mountains and 

 the Sierra Nevada for a long period, including certainly the whole of the 

 Tertiary epoch. Indeed, there are no Cretaceous rocks between the Wahsatch 

 Range and the crest of the Sierra, so that, in all probability, the sea has not 

 had access to any part of the belt between the 112th and 120th meridians 

 since the close of the Jurassic epoch. The extent of this belt over w'hich no 

 marine Cretaceous or Tertiary is found, in a north-south direction, is not as 

 yet exactly known. It appears to. extend, however, some distance to the 

 north of the boundary of California. We have, then, two enormous areas, 

 to one of which the sea has not had access since the close of the Jurassic 

 epoch, and to the other not since the end of the Cretaceous. The depres- 

 sions throughout these regions have, therefore, been so situated as to become 

 the recipients of detrital accumulations, either of subaerial origin or lacustrine 

 in chai'acter. The litholooical character of such materials would of course 

 vary considerably in different portions of the regions, according to the nature 

 of the conditions presented by the areas of depression and the surrounding 

 elevated ranges. Of the accumulated masses of material much would natu- 

 rally remain concealed by more recent deposits; but in many places a large 

 amount of erosion has so exposed the strata as to furnish excellent chances 

 for examining them. The large lacustrine basins are filled with deposits of 

 detrital material which in places have a combined thickness of several thou- 

 sand feet. Since the geological age of these formations could only be made 

 out by the aid of fossils, it is fortunate that in the region west of the Rocky 

 Mountains they are frequently abundantly supplied with these essential 

 guides to the geological relations of the various members of the scries. Of 



