136 CONDITIONS OF DEPOSITION 



bands interbedded with the sandstones. The shales 

 belong to definite horizons, or, in other words, were de- 

 posited during a certain part of the period instead of 

 coarse sand like that which lies above and below them, 

 but within the area of observation the coarse deposits 

 do not pass laterally into the fine-grained ones. In any 

 wide area of deposition such as that with which we are 

 dealing, it is usual to find a considerable change in the 

 nature of the material deposited, except in the case of 

 oceanic deposits, the organic oozes and red clays which 

 are formed far from land and under circumstances that 

 vary but slightly over immense regions. The sandstones 

 with which we are dealing, however, must have been 

 formed near land, possibly to some extent on the land. 



The great scarcity of fossils throughout the series is 

 a very significant fact and, together with the character 

 of the rocks, makes it impossible to regard the latter as 

 of marine origin. Lacustrine conditions, for at least the 

 greater part of the sediments, are also out of the ques- 

 tion, because it is difficult to conceive of such a continu- 

 ous accumulation of coarse sand in a lake. The nature 

 of the rocks is such as to justify a comparison with the 

 deposits found to-day in desert regions, but there are im- 

 portant points of dissimilarity. A frequent occurrence 

 in deserts is the interbedding of soluble materials such 

 as limestone, gypsum and salt with the sand and mud 

 swept down from the hills by wind and rain. Their 

 presence indicates either that the region had no outlet 

 for its drainage, or that the slope towards the outlet 

 was so slight that the scanty rainfall did not suffice to 

 carry away the soluble material, which was therefore 



