118 THE DESICCATION OF LATER GEOLOGICAL TIMES. 



As we ascend in the geological series to higher groups, the difficulty of 

 separating tlie past from tlie present offers an obstacle to generalizing. It is 

 often impossible to say what is Pre-glacial and what Post-glacial. There has 

 been no sudden change in the Fauna wliich would afford a guide to the 

 classification of the more recent deposits. Some of the characteristic Pliocene 

 species seem to have lived on to quite a recent date ; and, although now 

 certainly extinct, it is not safe to say that some of them may not have be- 

 come so aluiost within the historical period. These points, however, are not 

 of vital consequence in connection with the present discussion. What is 

 particularly desired here is, proof of an area of water surface over the west- 

 ern portion of this continent diminishing with the progress of geological 

 time, wliich is not difficult to procure ; and proof also that tliis diminution 

 has been the result of climatological as well as orographic causes, this latter 

 branch of the inquiry being by no means as simple as the other. 



It seems clear, however, that, so far as the region west of the Eocky 

 Mountains is concerned, if there was not much less water surface on the 

 whole during the later Tertiary times, there were, at least, no such great 

 lacustrine areas as existed during the Eocene and Miocene epochs. Lakes 

 Bonneville and Lahontan were large, Init not in comparison with Ute and 

 Gosiute lakes. So, too, the Pliocene lake-areas would not compare in size 

 with the bodies of water which preceded them in the same region. The 

 largest unbroken area of Pliocene lake surface west of the Rocky Mountains 

 seems to have been in the North Park region, occupying nearly the whole 

 of that valley, and extending along the North Platte up to latitude 41° 30' or 

 farther. Here, resting on the Azoic rocks, is a thickness of a thousand feet or 

 more of sandstones of varj'ing degrees of coarseness capped witli marls, the 

 whole of which Ibrmation is referred by Mr. King to the Pliocene, although 

 fossils seem to be wanting. Much is of course concealed by the later Post- 

 pliocene or Quaternary deposits. 



Beyond, a very considerable extent of surface was no doubt occupied by 

 water during the Pliocene epoch ; but the deposits of that age being so 

 largely covered with later ones, tlie exact limitation of the Pliocene lake- 

 areas cannot possibly be indicated. The present Humboldt Valley seems to 

 have been one of these old basins, and this probably had ramifications extend- 

 ino; noi'th and south between the rano-es, so as to form an extensive water 

 surface of complicated outline like that of the former Lake Lahontan. Some 

 few fossil i-emains were found in this region, indicating the Pliocene age of 



