984 REPORT OF NATIONAL MUSEUM, 1895. 



the periods when the lake waters had temporarily retreated— a hypoth- 

 esis that was confirmed by the finding of some fragments of onyx in 

 the upper travertine beds. 



"At the head of the ravine the travertine beds end abruptly in an 

 escarpment, beyond which one descends rapidly 500 feet through wind- 

 ing ravines, between sharp, jagged ridges of a metamorplnc rock, to 

 the bed of the Tule arroyo, a winding, yshaped gorge which runs north- 

 ward about 10 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 eftervescent 

 spring, known as the Yolcan, 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. 

 [See Plate 5.] 



" 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 diflferential 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 impassable except to foot travelers, and which are in most strik- 

 ing topographical 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 wash- 

 ing during the months immediately following the spring rains. At 

 other seasons there is not enough water to support life. They are com- 

 posed of distinctly stratified sedimentary beds standing on edge and 

 striking northwest and southeast, but which are so highly metamor- 

 phosed and so blackened and splintered by the weathering of this arid 

 region that their original character can no longer be determined. They 

 are mostly dark siliceous slates and fine-grained mica-schists. Some 

 beds have all the external appearance of limestones in their granular 

 structure and thin white veins, but their present composition shows 

 no trace of lime and is almost entirely siliceous. They are traversed 

 by well-defined dikes, which are also intensely altered. 



"Among the more striking rocks in this metamorphic series, at the 

 northern limits of the area observed, was a fine-grained hornblende 

 rock which microscopic examination shows to be properly an altered die- 



