FEAGMEI^TAL VOLCA^NIC EOCKS— CONGLOMERATES. 77 



pletely the notion that these fragf lents have been hurled into their present 

 positions by the explosive energy at the vents. 



Scoriaceous or slaggy fragments, "volcanic bombs," and the many forms 

 which lava takes when the blast from the crater carries up portions of -the 

 liquid and scatters them round the surrounding cone, are not found in the 

 conglomerates — at least I have never observed them. I will except from 

 this statement, however, one locality in the southern part of the Sevier 

 Plateau, where a profound gorge (named Sanford Cafion) gives a brief 

 exposure of what seems to have been an ancient trachytic vent subse- 

 quently buried by massive outflows, and which is composed chiefly of cinV 

 ders. This can hardly be called a conglomerate, however. The fragments 

 of the true conglomerates are apparently pieces of massive lava, just such as 

 are riven by the frost and other agencies of secular decav from cold rocks 

 in situ. Very many of them show more or less weathering or corrosion of 

 their surfaces, and very many do not indicate a trace of such action beyond 

 a slight discoloration. That these fragments have been broken from mass- 

 ive rocks is too patent to admit of question. 



The only explanation of the origin of the conglomerates which does not 

 involve us in absurdity is that they are derived from the waste of massive 

 volcanic rocks under the normal processes of degradation manifested in all 

 mountainous regions. While active vents usually throw out fragmental mat- 

 ter in great quantities, and while some of the fragments may have been thus 

 derived, yet I conceive that this process has contributed but an insignificant 

 portion of the entirety of the conglomerates. In the chapter on the Sevier 

 Valley and its alluvial conglomerates, I shall describe the process, now in 

 visible operation, by which beds of a similar nature are accumulating at the 

 present day upon a scale of magnitude not inferior to that which produced the 

 colossal formations now seen in the palisades of the plateaus. Throughout 

 the valleys which intervene between the ranges of plateaus fragmental beds 

 are accumulating in vast masses High up in the tabular ranges the frosts, 

 rains, and torrents are gradually breaking up, not only the anciently-out- 

 poured masses of lava, but also the older conglomerates, and are bearing 

 down through the great ravines and gorges the debris torn from the rocks, 

 and are scattering them over the valley plains in the form of very depressed 



