78 GEOLOGY OF THE HIGH PLATEAUS. 



alluvial cones, so flat or gently sloped that the conical form is not at first 

 recognized by the eye. Each cone has its apex at the gateway of some 

 mountain gorge, while its base is several miles out in the middle of the val- 

 ley. These cones are so broad and numerous, that they are confluent at 

 their bases and give the general impression of a very gently undulated 

 surface of alluvium covering the entire expanse of the valley. Could we 

 see them in vertical cross-section, we should find them to possess a well- 

 marked stratification agreeing with the stratification of the older conglom- 

 erates. A few fortunate exposures have here and there revealed their 

 internal structure, and a careful comparison leaves little doubt that the val- 

 ley alluvium and the ancient conglomerates were formed in substantially 

 the same manner and by the same process. 



If it be true that these conglomerates have been derived from the sec- 

 ular decay of massive eruptive rocks, of which the debris have been carried 

 down the old moimtain slopes by running water and stratified in great 

 beds of alluvia, then we may expect to find certain correlated facts, of 

 which the following are examples: (1.) We should expect to find these con- 

 glomerates grouped around ancient eruptive centers still preserving rem- 

 nants of the massive rocks which are presumed to have furnished the mate- 

 rial of the conglomerates. (2.) We should also expect to find that these 

 remnants consist of rocks of exactly the same varieties as we find in the 

 fragments of the conglomerates ; provided, however, that eruptions from 

 these centers subsequent to the formation of the conglomerates have not 

 completely overflowed and hidden the older outbreaks. (3.) We should 

 expect to find the loftiest portions or crowning summits of the plateaus to 

 consist not of conglomerates, but of massive rocks ; unless, indeed, the rela- 

 tive altitudes of the two classes of rocks has been reversed or modified by 

 subsequent upheavals or sinkages. 



The general idea here conveyed is that the process which formed the 

 conglomerates consisted in the transportation of fragmental matter from 

 high-standing ancient volcanic piles to low-lying plains and valleys around 

 their bases or along their flanks. These relations, 1 think, are very satis- 

 factorily shown after a careful analysis of the facts. We may still discern 

 the more important ancient eruptive centers with the conglomerates grouped 



