214 GEOLOGY OF THE HIGH PLATEAUS. 



on the east and the Tiishar on the west. Farther northward to the Juab 

 Valley a similar relation prevails. So far is the entire trough of the Sevier, 

 except at the barriers, from being due to erosion, that its floor has been 

 built up by the growth of alluvial formations of considerable magnitude. 

 They are of special interest because of the light they throw upon an inter- 

 esting problem in dynamical geology. 



THE FORMATION OF CONGLOMERATES. 



There are several kinds of conglomerate, formed by processes which, 

 though they may have some features in common, are on the whole strik- 

 ingly different. Glacial drift, though it undoubtedly falls within the usual 

 conception of a conglomerate, has an origin wholly different from that of a 

 littoral or alluvial conglomerate. Yet in respect to the source from which 

 its materials are derived — the disintegration of the harder rocks by water 

 and frost — the distinction is not well marked. The great difference is in 

 the methods and agents of transportation and final distribution. Alluvial 

 conglomerates agree with the littoral in having the same origin for their 

 materials, and the same transporting agent, moving water, but the two dif- 

 fer in respect to the conditions under which the transporting power is exer- 

 cised and the materials distributed. Thus these three kinds have some- 

 thing in common and each has some features peculiar to itself 



Sources of materials. — The stones and pebbles included in these forma- 

 tions are derived from the break-up of the hardest classes of rocks, which 

 are usually metamorphic or volcanic. Ordinary sandstones, limestones, 

 and clays, and shaly rocks in general seldom contribute to the mass of 

 fragments found in conglomerates. Attrition, weathering, and solution 

 utterly destroy them before they reach a resting-place. A few remnants 

 of rock not usually reckoned as metamorphic nor volcanic are some- 

 times inclosed, but they come from sedimentary strata as hard and endur- 

 ing as the others, and such strata are rare. Hard masses, originally con- 

 tained in softer beds, are sometimes found, but they owe their preservation 

 to their excessive durability, such as the flints of chalk, the chert, and many 

 forms of amorphous silica occurring in limestones. The localities from 

 which the stones come are no doubt very near those where they are 



