510 EMMONS AND MERRILL — SKETCH OF LOWER CALIFORNIA. 



is not enough water to support life. They are composed of distinctly 

 stratified sedimentary beds standing on edge and striking northwest and 

 southeast, but which are so highly metamorphosed 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 silicious 

 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 en- 

 tirely silicious. They are traversed by well defined dikes w^hich are 

 also intensely altered. A specimen from one dike was examined micro- 

 scopically and found to be a metamorphosed clastic, composed of irregu- 

 larly notched quartz granules with sporadic patches of pale green horn- 

 blende and considerable infiltrated calcite. 



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 probably an altered diorite. 

 The groundmass consists of an aggregate of plagioclase feldspar, ap- 

 parently anorthite, with fibrous hornblende containing inclusions sug- 

 gestive 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 examination shows the groundmass 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 su23posed 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 Bluff 

 point. It has an elevation of about 3,500 feet, and overtops all the 

 highest sum^mits within a radius of 15 to 20 miles, thus offering an ad- 

 mirable point of view from which to study the physical structure of the 

 region. The basalt cap has an aggregate thickness^of 500 feet, and con- 

 sists of an upper layer of dark vesicular olivine-bearing rock 350 feet in 

 thickness resting on 150 feet of gray, fine-grained rock containing abun- 

 dant large crystals of olivine. The upper layer has a dark smoky glass 

 base with the usual microlites of feldspar and augite and small pheno- 



