GEOLOGICAL STRUCTURE — EASTERN RANGE. 509 



The winding bed of the modern stream cuts into the travertine de- 

 posit, exposing at one place a chif of over 20 feet in height, showing 

 three distinct layers of Mexican onyx, one of which is over three feet 

 thick, interstratified with the travertine, while for a distance of nearly a 

 mile down the ravine sheets of the more resisting onyx cap the little 

 travertine mesas on either side. The occurrence of the onyx, which is 

 a thermal spring and surface deposit, in successive layers, separated by 

 travertine and resting on conglomerate, indicates a probable successive 

 rise and fall of the waters of the lake where the travertine was de- 

 posited, which would have admitted of some slight erosion of the deposit 

 in the periods when the lake Avaters had temporarily retreated, a hy- 

 pothesis that was confirmed by the finding of some fragments of onyx 

 in the uj^per travertine beds. 



At the head of the ravine the travertine beds end abruptly in an escarp- 

 ment, beyond which one descends rapidly 500 feet through winding 

 ravines, between sharp, jagged ridges of a metamorphic rock, to the bed of 

 the Tule arroyo, a winding, V-shaped gorge^ which runs northward about 

 ten miles, then northeastward to the gulf of California, draining the 

 whole region east of the divide. At one point this gorge widens out into 

 quite a valley, in which are travertine deposits about 50 feet in thickness, 

 with layers of onyx in the upper part. Relics of the thermal action are 

 found at the present day in a little effervescent spring, known as the 

 Volcan, which issues from the top of a dome-shaped mound of calcareous 

 tufa in the narrow bottom of the ravine before it opens out into the valley 

 containing the travertine deposits. 



These travertine deposits are entirely isolated and have no present 

 connection with those of the interior valley to the east of the divide, 

 their level being about 400 feet lower than the divide where the nearest 

 lake-bed deposits end. The similarity of their composition, their rela- 

 tions to the underlying rocks, to the onyx formation, and to modern 

 erosion all suggest, however, a common origin with the lake beds, and, 

 if once connected with them, there must have been a differential move- 

 ment since their deposition which produced the present difference of 

 level. 



Beyond the Tule arroyo to the eastward arise a series of sharp, jagged 

 peaks, which attain a maximum elevation of about 3,000 .feet, deeply 

 scored by a most intricate system of deep, winding ravines, quite im- 

 passable except to foot travellers, and which are in most striking topo- 

 graphical contrast to the level valleys and plains of the region west of 

 the divide. Within these hills at various points are placers from which 

 the Mexicans obtain considerable coarse gold by dry washing during the 

 months immediately following the spring rains. At other seasons there 



