224 



GEOLOGY OF THE UIGH PLATEAUS. 



at considerable distances from the orifices. Want of opportunities for ob- 

 serving such formations of unquestionable origin prevents me from having 

 any just conception of the nature, extent, and texture of such accumula- 

 tions. But it seems sufficiently clear that there could be no difficult}^ in 

 distinguishing them from such as are with equal certainty attributable to 

 sub-aqueous or alluvial deposition. I have observed but few exposures 

 which I can attribute to such an origin. That the great mass of conglom- 

 erates of the High Plateaus were accumulated from the debris derived from 

 the erosive destruction of volcanic beds cannot be doubted. The only 

 question is whether they are alluvial or sub-aqueous, and of the former 

 origin I entertain no doiibt. The fragments seldom fail to reveal traces of 

 attrition and weathering, never preserving sharp angles like those pro- 

 duced by fresh fracture. But, on the other hand, the attrition is not 

 ordinarily extreme. In most cases there is enough of it to indicate dis- 

 tinctly that the fragments have really been abraded, though with no 

 great loss of substance. The stones of sub-aqueous conglomerates, on 

 the contrary, are almost always much worn and rounded. Again, the 

 sizes of the stones range from a fraction of a cubic inch to several cubic 

 feet ; in rare instances to more than a cubic yard. 



In whatsoever manner we compare the great conglomerates now form- 

 ing solid rock masses and uplifted as plateaus with the alluvial conglom- 

 erates now forming in the valleys, we cannot fail to be impressed with the 

 evidence that both were formed by essentially the same process. The only 

 differences of any appreciable moment which are now discoverable arise 

 from the fact that the older conglomerates have been consolidated into rock- 

 masses, while the later ones have not. 



