Deposition in the Deserts. 169 



surface had the character of a widely extended and approximately 

 horizontal plain. If we now look around on the earth's present 

 surface for regions in which the freshly formed deposits with 

 horizontal planes are heing formed, we find them, in the first 

 place, at the bottoms of seas and large inland lakes. In them 

 are formed deposits with horizontal surfaces — that is to say, 

 stratified deposits. 



Now, it is ver}^ important to note that besides the sea bottom 

 there exists another class of regions of the earth's surface on 

 which the products of denudation are spread with great regu- 

 larity over wide horizontal areas. These are the deserts and 

 steppes. Both therefore are areas which must not be left out of 

 view in the discussion of the origin of stratified deposits. Strati- 

 fication does not all originate under water. 



The activity of denudation is a double one. It destroys the 

 rocks of the earth's surface and transports the comminuted ma- 

 terial from its place of origin. In our regions it is water that 

 destroys the rocks ; it dissolves them chemically and frost fissur- 

 ing comminutes them mechnically. Water is also in our lati- 

 tudes the most important transporting agency. In the desert it 

 rains but seldom. The time in which water may there destroy 

 rocks and transport debris is at most 65 days in the year. It 

 has been thought that during the remaining 300 days denuda- 

 tion in the desert is at a standstill ; yet careful, unbiased study 

 teaches that in these 300 dry days denudation is intense. A 

 burning sun beats down on the rock surface, unprotected by any 

 plant cover. In Texas daily variations in temperature of 40° C. 

 are not at all rare ; and large and small stones are cracked by 

 the heat. Often have I picked up the halves of such cracked 

 pebbles still fitting together. In Texas I saw granite blocks as 

 high as houses divided by wide cracks, and Mr von Streeruwitz 

 told me that he had seen and heard the cracking of such blocks. 

 The variously colored constituents of the granite become heated 

 to different degrees and fall apart in the form of coarse gravel. 

 In a valley of the Sierra de los Dolores a rainfall had filled the 

 rocky bottom of the valley with granite gravel to a depth of 3 

 feet ; this gravel had been formed by insolation on the granite 

 rocks in the course of years. Deep caves weather out of the 

 granite wherever the water remains longer, and these increase 

 the mass of the products of denudation. 



