1893.] NATURAL SCIENCES OP PHILADELPHIA. 341 



early spring, and high up in the mountain rains are frequent during 

 the spring and summer. In nearly all the canons there are ever- 

 flowing springs which furnish a stream of cold water ; but this dis- 

 appears near the exit of the mountain stream and beyond the 

 mountain the valley is a broad bowlder-strewn channel carrying 

 water only spasmodically. A rain in the mountains suddenly fills 

 the channel with a seething torrent, which advances with a solid 

 front like a wave, and the force of these floods is shown by the large 

 pebbles and small bowlders of the mountain limestone in the stream 

 channel, many miles from the mountain. 



That the valley is being rapidly deepened in the mountain is 

 proved by the occurrence of a cemented conglomerate, a river bed 

 conglomerate, clinging to the sides of the canon at an elevation of 

 fifty feet above the present channel. A similar conglomerate is 

 now forming in the channel. Most of the annual erosion is done in 

 a very short period of time, since during most of the year only a 

 small, clear stream of spring water flows for a short distance. Dur- 

 ing the remainder of the year the down cutting is chiefly done by 

 corrosion and this mainly in the upper course. The stream is 

 strongly impregnated with lime, and as it descends and is subjected 

 to evaporation the lime solution becomes supercharged and a tufa- 

 ceous deposit is formed. Thus in the lower two or three miles the 

 stream flows in a cemented pebbly bed while above this it flows 

 chiefly on bed rock in a corroded valley. The tufaceous deposit 

 encloses vegetable remains as well as pebbles. 



Corrosive work is well shown in the hillsides, for the compact 

 limestone is ridged and furrowed into an almost perfect imitation, 

 in miniature, of mountain structure of accentuated topography. 

 So roughened is the surface from this cause that the rock has the 

 feeling to the feet and hands of a slaggy lava flow, and so perfect 

 is this resemblance that the ranchmen compare it with glass and 

 call it porphyry and granite, which led me to expect to find eruptive 

 rocks in the mountains. 



(c) Quaternary Conglomerate. — Along the eastern base of the 

 mountains, extending well out on the plateau, is a conglomerate 

 composed of mountain pebbles and often cemented with a tufaceous 

 cement. This deposit is frequently many feet in thickness and 

 rests unconformably on the Carboniferous. As the source is 

 approached, that is near the mountain base, the fragments become 

 larger and more angular. It has been inferred by some that this 



