 



ALLUVIAL CONGLOMERATES. 



223 



inspection of favorable exposures we may well believe; yet it is highly 

 probable that the two kinds would be confounded on a hasty examination, 

 and the distinction would be difficult to verify even by careful study, unless 

 the exposures were extensive and conspicuous enough to display very fully 

 and clearly their respective characters. These doubts generally would 

 prevail in those cases where a decision would have to turn only upon the 

 intimate structures of the deposits. Collateral circumstances, however, 

 may often decide the question. 



Throughout the volcanic portions of the District of the High Plateaus 

 the conglomerates are present in prodigious masses. They constitute a 

 large proportion of the rock masses of the plateaus, and form many miles of 

 escaipment more than a thousand — sometimes more than 2,000 — feet in 

 thickness. In the central and southern portions of the plateaus they can- 

 not fall much short of one-half of the masses now open to observation, and 

 taking the volcanic portion of the entire district, a rough estimate would 

 place their volume at least at a third of the whole eruptive material. They 

 are well stratified, and though the distinctness of the bedding is somewhat 

 variable, the stratification never becomes obscure. Indeed, on the whole, 

 these conglomerates seem to be about as well stratified as the average of 

 those which are attributed to sub-aqueous deposition. The individual beds 

 are not so thick and massive and show partings more frequently or at 

 shorter intervals. 



The occuiTence of large stratified accumulations of pyroclastic mate- 

 rials in regions or districts which have been the theaters of protracted vol- 

 canic activity is a fact of common observation. They abound throughout 

 the State of Colorado and along the more or less volcanic ranges of North- 

 ern Wyoming, Montana, and Idaho. They excited the admiration of 

 Scrope in Central France, and are conspicuous in Sicily and around Vesu- 

 vius. Indeed, every volcanic region will doubtless be found to display 

 them to a greater or less extent. Where large bodies of water wash the 

 flanks of volcanic mountains and ranges we may expect to find large 

 bodies of sub-aqueous conglomerate formed from their debris. Volcanic 

 tuffs are formed by the mechanical projection of dust, ash, rapilli, and 

 small fragments from vents blowing out gases and steam, and falling 



