788 DESCRIPTIVE GEOLOGY. 



in the long, flat spurs, are largely concealed by suriace accumulations. In 

 the pass through which the wagon-road passes, the rocks seem to have a dip 

 to the eastward. The easternmost mass closely resembles the trachyte 

 above described, having yellowish, decomposed feldspars and dark-brown 

 hornblendes in a rough-feeling groundmass, in which also small undecom- 

 posed plagioclases can be distinguished. 



A yellowish-brown rock, having a conchoidal fracture, and less of the 

 trachytic feel than the last described, forms the main mass of the peaks. 

 Through the homogeneous-looking feldspathic groundmass are plentiful 

 crystals of rather vitreous-looking feldspar and yellow-brown decomposed 

 hornblendes. Under the microscope, the feldspars are seen to be mostly 

 plagioclase, and the decomposed hornblendes present interesting peculiari- 

 ties, which are described in the microscopical report. Apatite, which is 

 found in all these rocks, occurs in broken prisms, presenting a peculiar ar- 

 rangement of dust-like material in their interior, as shown in Vol. VI, Plate 

 I, fig. 9. 



These rocks present a peculiar type, being quite different in lithologi- 

 cal habit from any other andesites observed in the region of the Survey, and, 

 on the other hand, being distinctly trachytic in external character. In this 

 connection, it is interesting to note the occurrence of another sanidin-bearing 

 andesite in little isolated hills in the open region to the north of the Kamma 

 Mountains, near the Indian Spring. It resembles the first of those described 

 above, being of a dark greenish-gray color, and showing large sanidins and 

 decomposed hornblende, but the groundmass is more compact and close- 

 grained. On its weathered surface is a coating of reddish decomposed rock, 

 containing silica in thin films. 



At the northern end of the andesite hills are outcrops of purple, finely 

 laminated, slightly calcareous argillites, resembling those of the range next 

 east, which have been, like them, referred to the Jurassic formation. 

 From these, in a little side-ravine just south of the northern branch of the 

 wagon-road, issues a little spring of remarkably pure cold water, called 

 Lander Spring. 



The northern group, which is somewhat lower than the two just de- 

 scribed, is entirely made up of rhyolites and rhyolitic breccias and tufas, 



