GEOLOGY AND NATURAL HISTORY OF LOWER CALIFORNIA. 985 



rite. The ground mass consists of an aggregate of plagioclase feldspar, 

 apparently auorthite, with fibrous hornblende containing inclusions 

 suggestive of interpolations of hypersthene and diallage. Associated 

 with this was a grayish massive rock thickly studded with short, stout 

 crystals of black hornblende 2 to 5 millimeters in diameter and 5 to 8 

 millimeters in length. Microscopical examinations show the ground 

 mass to be a granular aggregate of almost colorless augites with a few 

 plagioclase feldspars, and the rock apparently belongs to the group of 

 hornblende-pyroxenites of Williams. When collected, these rocks were 

 supposed to be interstratified with the metamorphic series, as their 

 outcrops had the same general strike; the result of microscopical 

 examination indicates that they are probably altered intrusive sheets. 



"The flat- topped ridge of mesa sandstones south of the ravine in 

 which the principal New Pedrara onyx deposits occur is thickly strewn 

 with subangular blocks of augite-andesite, which have apparently 

 weathered out as the soft ash of which the beds are composed has 

 worn away. At the eastern extremity of this ridge, on the very crest 

 of the divide, is a high basalt-capped mesa, nearly a mile in diameter 

 called by us Bhifl' Point. It has an elevation of about 3,500 feet, and 

 overtops all the highest summits within a radius of 15 to 20 miles, thus 

 offering an admirable point of view from which to study the physical 

 structure of the region. The basalt cap has an aggregate thickness of 

 500 feet, and consists of an upj)er layer of dark vesicular olivine- 

 bearing rock 350 feet in thickness resting on 150 feet of gray, fine- 

 grained rock containing abundant large crystals of olivine. The upper 

 layer has a dark smoky glass base with the usual microlites of feldspar 

 and augite and small phenocrysts of augite, olivine, and feldspar. 

 Between these flows a zone of decomposition several feet in thickness, 

 colored brilliant red by peroxidation of the iron, makes a prominent 

 line, visible from a great distance, on the bluff faces which almost com- 

 pletely surround the mesa. 



"The surface of the mesa has a gentle slope westward and ends to the 

 eastward in an almost perpendicular escarpment overlooking the Tule 

 arroyo 1,500 feet below, which has here widened out into a consider- 

 able valley that drains the northern slopes of the White range far to 

 the south. Beyond this valley, i^artly cutting off' the view of the Gulf 

 of California, lies the group of dark rugged peaks of metamori)hic 

 slates called the Volcan Peak group, which the Tule arroyo almost 

 completely encircles in its circuitous course to the sea. Through the 

 gaps in this range can be distinguished the pale blue waters of the 

 Gulf of California and occasionally portions of the coast line, as well 

 as several of the group of small islands which lie a few miles off' the 

 shore in this latitude, and whose abrupt outlines show them to be 

 probably j^rojecting points of the buried metamorphic ranges. 



"The arms of the interior valley, which lie to the south and west of 

 the Bluff Point mesa, have a floor of granite which is entirely denuded 



