506 EMMONS AND MERRILL — SKETCH OP LOWER CALIFORNIA. 



range, on the other hand, are composed largel}^ of eruptive rocks and 

 appear to be projecting portions of the older mountains laid bare by 

 erosion, but in a few cases are mesa-topped ridges capped by horizontal 

 lava flows of later age than the mesa sandstones. 



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, about 20 miles north of the line of the section has a re- 

 markably 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 erosion had 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 w^estern range at the ruined mission of 

 San 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 of section are drained by deep and 

 narrow arroyos of more recent formation into the gulf of California. 

 The present bottoms of the valley are occupied by recent deposits of 

 porous limestone or travertine and coarse conglomerate with calcareous 

 cement containing rounded fragments of both eruptive and sedimentary 

 rocks in great variety and of varying size up to several feet in diameter. 

 The evidence of wells which get water in the lower parts of the valle}^ 

 at 40 to 60 feet below the surface, and of adjoining mesas in the valley 

 which afford partial sections show a present thickness of little over 100 

 feet of these beds, but their elevation in shallow ravines, 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 hun- 

 dred 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 position of the deposits indicate that they were laid 

 down in an enclosed body of water, probably an interior lake, of com- 

 paratively recent date. AVhat reriiains of these beds barely serves to 

 smooth over the inequalities of the underlying mountains whose com- 

 ponent rock masses often outcrop across the stream beds, especially along 

 the eastern portion of the valley. Even where there is no actual outcrop 

 the appearance of frequent fragments of granite or sedimentary rocks, as 



