780 DESCRIPTIVE GEOLOGY. 



portion of the main ridge, where they have broken through the granite, 

 and spreading out thence to the north, east, and south, in the latter direc- 

 tion formed a southern continuation of the mountains, in a narrow ridge, 

 set a little to the eastward of the main line of elevation. Through the 

 rhyolites at various points, but generally in the line of the main crest, have 

 broken out flows of basaltic rock, now found overlying the former, and 

 capping the higher peaks at either extremity of the range. 



These rhyolites present a most interesting series, showing types of all 

 the principal varieties, from a compact porphyritic, almost granitic, structure, 

 unusually rich in black mica, with earthy, porous varieties, without distinct 

 crystals, to pearlites of most varied appearance, from those containing mica 

 and occasionally other crystalline ingredients, to the most remarkable vitre- 

 ous pearlites, without crystals, made up of sphserulitic concretions, and car- 

 rying in their mass rolled pebbles of the most pure black obsidian, accom- 

 panied by various chalcedonies and white pumiceous tufas and breccias. 



The southern portion of the mountains, which seems to be an indepen- 

 dent and later formation, is a steep narrow ridge, gradually descending to 

 the south and west, with a few prominent breaks in its continuity, until it 

 disappears beneath the Quaternary soil of the desert plains. It is princi- ' 

 pally composed of rhyolite of a rather massive habit, which weathers into 

 tabular masses, with a dark reddish-brown surface, and forms abrupt escarp- 

 ments to the north and west, its jointing planes having a general eastern dip, 

 in some cases as high as 30°. It would seem as if the mass had been faulted 

 up perhaps at the time of the basaltic eruption. The main rock of Aloha 

 Peak, the culminating point of the ridge, is a porphyritic rhyolite of rough 

 trachytic texture and reddish-gray color, showing glassy feldspar and a little 

 mica as macroscopical crystals. In the felsitic groundmass, a tendency of 

 the red and gray portions to arrange themselves in thin bands gives a striped 

 appearance to the rock. On the dark-brown weathered surfaces of the 

 cliffs, patches of bright lemon-yellow lichens present a pleasing contrast of 

 color to the general sombre appearance of these volcanic rocks. Under the 

 microscope, the ingredients of the rock are seen to be sanidin, frequently 

 in Carlsbad twins, a little plagioclase and quartz, all with glass-inclusions, 

 together with biotite, partly fresh and partly altered into a brown granular 



