272 



NATURE 



{Jan. 23, 1879 



, THE GEOLOGICAL HISTORY OF THE 

 COLORADO RIVER AND PLA TEA US » 



II. 



IN the Pliocene period the climate of the region gradu- 

 ally experienced a great change. Miocene times were 

 characterised by a moist and ordinary sub-tropical climate ; 

 the Pliocene by dereloping an arid one, like that which 

 now prevails there. Let us look at the causes which make 

 this climate what it is. In whatever rectilinear direction 



we may undertake to pass fromjjthe Pliocene Country to 

 the ocean we shall be compelled to cross some of the 

 loftiest barriers of the Continent. It is hemmed in by 

 range after range of high mountains. The winds laden 

 with moisture are wrung dry long before they reach the 

 plateaux in the heart of the province. The prevailing 

 wind throughout the year is from the westward, and must 

 cross the Sierra Nevada. Sweeping across the great 

 basin it blows over many ranges, and at last strikes the 

 Wasatch and the chain of high Plateaux which form the 



-Grand Cancn, fi-_m Hie Middle Ter ace. 



western" »vall cf Ihe Plateau Province. Here it is 

 suddenly projected upward more than a mile and flings 

 down moderately copious rains. Descending into the Cliff 

 and Canon Country, its humidity is so much exhausted 

 that it can yield but the scantiest pittance of snow and 

 showers. Thus the country is a desert. Now the strange 

 forms' impressed upon this land— its cliffs and canons, 



' By Capt. C. E. Dtifton.'U.S. Army, A's'start-Geo'ogistU.S. Si rvsy of 

 Ae Reeky Mountain iRegion, un'ler Prof. J. W. Powell, in charge. Con- 

 tinued from r 25^. 



with their myriads of wonderful shapes and their astound- 

 ing architecture — are due, as we shall presently find, in 

 great part to the aridity. The aridity is due to the great 

 barriers which surround it, and above all to that great 

 barrier of high plateaux which lies upon its western 

 verge. Here, then, Ave may look for another key which 

 may unlock another door within the vestibule. The 

 search will not be fruitless. 



The district of the high plateaux has been during the 

 last four years a field of special study by myself, and has 



