794 DESOEIPTIVE GEOLOGY. 



on the edge of the desert, and the building of two small mills, to work the 

 so-called ore Had the vein, this bed of decomposed basalt, been metal- 

 bearing, it might have been the source of untold wealth, since, with a thick- 

 ness of 40 to 60 feet, it could be traced almost continuously over an extent 

 of thousands of acres ; but a careful chemical examination revealed, as might 

 have been expected, no metal other than a small percentage of iron. 



The extreme southern point of the mountains is a rounded hill, rising 

 about 500 or 600 feet above the desert, formed entirely of basalt, which, 

 from the contrast of its blackened weathered surface with the white desert- 

 plains, has received the name of Black Rock, afterward transferred to the 

 whole ridge. It is composed of a fine- grained rock, on freshly broken sur- 

 faces of a greenish-gray color, a conchoidal fracture, and sometimes rather 

 granular texture, showing small crystals of fresh plagioclase and abundant 

 yellow-brown augites ; it contains also considerable calcite, which fills 

 crevices, and sometimes forms minute crystals in the mass. From a little 

 hill just north of this, at the southern end of the main ridge, was obtained 

 a remarkably interesting dolerite, which was found alternating with a red- 

 dish, porous, fine-grained basalt. The dolerite is remarkable for its large 

 tabular crystals of plagioclase, sometimes an inch in diameter. It is of a 

 dark greenish-gray color, and resembles that found at the southwest end 

 of the Kamma Mountains, but is much more coarsely-grained, and has a 

 somewhat resinous lustre. In it can be distinguished crystals of dark-brown 

 augite up to one-fourth of an inch in diameter. Under the microscope are 

 detected, besides plagioclase and augite, olivine and magnetite, but no 

 sanidin, quartz, or titanic iron. The crystalline ingredients are generally 

 very fresh and unaltered, and a little amorphous base, in the form of 

 wedge-shaped grains between the crystals, is present. 



Basalt forms the mass of the hills for some distance north of the Black 

 Rock. Among these basalts was found a deposit of basaltic tufa, a loose, 

 gray, fine-grained mass, not to be distinguished by the unaided eye from 

 that observed in the Pah-tson Mountains, but which, under the microscope, 

 is seen to be made up, with the exception of some tabular crystals of feld- 

 spar, of fragments of the hyaline volcanic glass, called palagonite. The 

 occurrence of palagonitic tufa here is particularly interesting, as being the 



