542 PRINCIPLES OF STRATIGRAPHY 



origin of all formations should be considered axiomatic, except 

 where fossils indicated fresh water, in which case fresh-water lakes 

 or inland seas of any desired magnitude were postulated as the 

 medium in which these formations were deposited. Now, how- 

 ever, that we have the extensive observations on the subaerial origin 

 of formations of great thicknesses, embodied in the epoch-making 

 works of Walther and others, there is no longer any excuse for ac- 

 cepting the time-worn marine or fresh- water lake origin of all the 

 sedimentary formations. It becomes more and more apparent that, 

 where formerly marine conditions were regarded as the only pos- 

 sible ones for the, deposition of certain formations, desert con- 

 ditions are often the more probable explanation, while formations 

 generally referred to a lacustrine origin prove in many cases to be 

 subaerial deltas and flood-plain deposits of streams or the accumu- 

 lation of wind-transported material upon a relatively dry land sur- 

 face. It is no longer one of the axioms of the stratigrapher that 

 all stratified deposits were laid down under water ; in fact, we must 

 be prepared to question the subaqueous origin of all formations that 

 do not show by the occurrence of either marine or fresh-water fos- 

 sils throughout that they were formed under water. The possibility 

 of the existence of unfossiliferous subaqueous formations is not ex- 

 cluded, and the subsequent destruction of the organic contents of 

 the formation must be considered. At the same time the possibility 

 of a subaerial origin of all unfossiliferous clastic formations, or of 

 those containing marine or fresh-water fossils only at great inter- 

 vals, must be borne in mind, while the occurrence of remains of 

 terrestrial organisms should be considered as prima facie evidence 

 of subaerial origin of the formations containing them until their 

 subaqueous origin is positively established. 



Texture of Atmoclastic Rocks. Among the characters of 

 atmoclastic rocks should first be mentioned their coarseness of 

 grain, especially near the mountains from which the material has 

 been obtained. In the talus region the material will not only be 

 coarse, but the blocks will for the most part be angular. Often the 

 structure is suggestive of morainal accumulations. (Walther-52 : 

 /oj.) Large and small, round and angular blocks abound in the 

 sandy ground mass with a total absence of sorting, the result of the 

 indiscriminate piling together of the material by the tremendous 

 flood rains of the desert. The evidence of violent impact is further 

 shown by the abundance of concussion marks and the total absence 

 of the woody vegetation which is ground to fragments and washed 

 away by the floods. Portions of the delta built by less violent floods 

 show interbedded, well-stratified, sandy layers, and these become 



