218 POOLE— -ON THE GREAT AMERICAN DESERT. 



Pinnacles also, either isolated or arranged in groups, stand out 

 of the rounded hill side or denuded plain, and resemble the roches 

 perches on pillars of ice on the great mers de glace of the Alps. 

 Professor Tyndal has clearly explained how the blocks of stone 

 became perched on pillars of ice on the surface of the me/'s de 

 glace. Similarly these pinnacles capped with blocks of sandstone, 

 more compact and less friable than the conglomerate that composes 

 their shafts, were doubtless formed. 



The harder and more enduring blocks of sandstone, fragments 

 of an overlying bed, have protected the more perishable red conglo- 

 merate from the disintegrating action of the heavy rains, while 

 the general level of the country was being reduced ; and these 

 pinnacles remained to mark its position in times gone by. 



Another interesting record similar in character to that preserved 

 by the " red buttes " is the Devil's Slide, as it is called, in Echo 

 Canon. Two parallel dykes of syenite some four to eight feet 

 thick and close together, rise abruptly to a height of twenty feet 

 above the surface of the mountain side. 



The extent of the effect of the subaerial agents on the constitu- 

 ents of mineral veins, in this region, furnishes still more conclusive 

 evidence of the great period of time which has elapsed since the 

 waters of the great lake receded from their more elevated levels 

 to their present position. 



In the beds of certain of the canons and on the mountain sides 

 where the strata are of such a nature as to retain the water, there 

 we yet find even at great elevations the constituents of the lodes as 

 sulphurets. But where by natural channels the water absorbed by 

 the ground in the wet season is allowed to drain away, the warm 

 air, containing of course carbonic acid, which filters in, has an 

 opportunity to act on the mineral substances contained in the vein, 

 decomposition is set up and oxidation of the sulphurets takes place 

 in a degree proportionate to the facilities offered by the structure of 

 the vein, and the tendency of the minerals to decompose under such 

 circumstances. 



Now since even down on the foothills, the outcroppings of 

 galena lodes are found to consist of cerussite, with here and there 



