GEOLOGY AND NATURAL HISTORY OF LOWER CALIFORNIA. 981 



" To the north the valley appears to grow wider, and out of its midst 

 rise a few conical peaks, the most prominent of which, known as San 

 Juan de Dios [Plate 4], about 20 miles north of the line of the section, 

 has a remarkably graceful outline and a probable elevation of over 

 4,000 feet. It is composed, in great measure, of eruptive rocks, among 

 which felsite, diabase, liparite, and basalt were recognized, while ero- 

 sion has disclosed on one side an underlying coarse quartzite. At its 

 base is one of the rare springs of the region. Similar peaks are seen to 

 rise out of the plain far to the northward at probable distances of 10 

 to 15 miles apart. 



" The slope of the broad stream beds in the lower part of the valley is 

 so imperceptible that the direction of its drainage is difficult to deter- 

 mine; but aneroid observations indicate that the portion examined is 

 drained through the gap in the western range at the ruined mission of 

 Sail Fernando (elevation about 1,800 feet), and thence probably by the 

 San Fernando River bed to the Pacific Ocean. Limited portions of 

 the eastern edge to the north of the line section are drained by deep 

 and narrow arroyos of more recent formation into the Gulf of Cali- 

 fornia. The present bottoms of the valleys are occui)ied by recent 

 deposits of porous limestone or travertine and coarse conglomerate 

 with calcareous cement containing rounded fragments of both eruptive 

 and sedimeutary rocks in great variety and varying size up to several 

 ieet in diameter. The evidence of wells which get water in the lower 

 parts of the valley at 40 to 60 feet below the surface and of adjoining 

 mesas in the valley, which afford partial sections, show a present thick- 

 ness of little over 100 feet of these beds; but their elevation in shallow 

 raviues — notably the one on the southeastern edge of the Buena Vista 

 plain, in which are the New Pedrara onyx deposits, and remnants of 

 calcareous conglomerates remaining on the flanks of the bounding 

 ridges at other points — indicate that the original thickness of these 

 deposits may have been several hundred feet, and that the greater part 

 has already been removed by erosion. No fossil evidence was obtained 

 as to their absolute geological age, but the character and iiosition of 

 the deposits indicate that they were laid down in an inclosed body 

 of water, probably an interior lake of comparatively recent date. 

 What remains of these beds barely serves to smooth over the inequali- 

 ties of the underlying mountains, whose component rock masses often 

 outcrop across the stream beds, especially along the eastern portion of 

 the valley. Even where there is no actual outcrop the ajipearance of 

 frequent fragments of granite or sedimentary rocks, as the case may 

 be, indicate that these rocks are to be found in place near by and not 

 far from the surface. In some cases the ground is whitened over con- 

 siderable areas by the abundant small fragments of vein quartz, result- 

 ing from tlie disintegration of the underlying slates. 



<' The relative age of the interior lake beds may be assumed to bear 

 some relation to that of the calcareous conglomerate already mentioned 



