DEGRADATION OP TOE TUSHAR— CONGLOMERATES. 



179 



scale have produced new features by uplifting the range en masse. But as 

 these recent movements apply to the whole uplift, the relative altitudes of 

 the loftier portion, which furnished the debris, and the less lofty portion, 

 which has received it, have not been much, if at all, changed with respect 

 to each other. But erosion has apparently effected what displacement has 

 not ; it has nearly equalized the levels of the two portions. The volcanic 

 masses near the foci must have been very voluminous, for the conglomer- 

 ates derived from them extend with great thickness over a large area, rival- 

 ing in bulk, if indeed they do not surpass, the enormous masses yet 

 remaining. Wherever we find strata composed of clastic materials, the 

 present methods of reasoning in geological science compel us to acknowl- 

 edge that they have been derived from the degradation of masses of even 

 greater magnitude.* In the case of a great sub-aerial conglomerate, formed 

 under conditions which are still existing and a process still operating, we 

 naturally look to the vicinity or border of the conglomerate itself for the 

 source of the materials. We find a very obvious source to the northward. 

 The structure of the great uplift of which the conglomerate forms a part 

 and large masses of eruptive strata in situ, composed of materials agree- 

 ing with those found in the clastic beds, confirm this view so strongly, that 

 there seems no room for question. But the mass of the conglomerate argues 

 an enormous degradation. To supply so vast an accumulation the older 

 eruptive area in the central part of the Tushar must have been piled thou- 

 sands of feet high with successive sheets no longer visible, or have been 

 the theater of eruptions separated by long intervals of erosion, which in 

 the long run removed the lavas as fast as they were erupted. A view which 

 is a compromise between these two I regard as decidedly preferable, and 

 most fully sustained by the general tenor of the evidence throughout the 

 entire district. We may look back to a peiiod somewhat earlier than Mid- 

 dle Tertiary, when the volcanic eruptions built up iEtna-like highlands of 

 eruptive materials, not by rapidly succeeding outpours, but by alternating 

 emission and quiescence. Between the outbreaks many years or centuries 

 may have elapsed, but the accumulation was much more rapid for a time than 



* Except in cases where pulverulent and fragmentary materials have been ejected and scattered, 

 which is not the case in the present instance. 



