OMBE MOUNTAINS. . 495 



This granite body of the Ombe Mountains covers only a limited area, 

 extending", however, across the entire width of the uplift, until the lower spurs 

 on both sides are concealed beneath the Quaternary deposits of the valleys. 

 It occupies the narrow, relatively-depressed portions of the mountains, 

 known as Patterson Pass, which rises above Tecoma Valley, on the west side, 

 with gentle slopes, some 2,400 feet, but falls away much more rapidly 

 toward the Great Desert to the eastward, which lies nearly 3,000 feet below 

 the water-shed of the pass. As thus defined, it measures about 4 miles in 

 width, but scarcely more than 2 miles in a north and south direction, being 

 overlaid by the heavy quartzite and limestone formations, which would 

 appear to form an anticlinal fold, dipping away in opposite directions from 

 the granite, the latter coming to the surface only along the axis of the fold. 

 This granite is a medium-grained rock, somewhat friable in texture, and of 

 a reddish-gray color, derived from an admixture of both red and white feld- 

 spars. The mineral constituents are chiefly quartz in small, translucent 

 grains, associated with both monoclinic and triclinic feldspars. Mica, in 

 thin brown flakes, frequently adhering to the broader faces of feldspars, is 

 present, but in subordinate amounts. The white feldspars are often an inch 

 in length, forming a strong contrast to the smaller, but more abundant, red 

 crystals. A determination of the alkalies in one of the large white feldspars 

 gave: soda, 2.34 per cent. ; potassa, 12.58 per cent. The other small out- 

 crops of granite iare found to the northward of Patterson Pass, in the limestone 

 body, where they occur as irregularly-shaped masses and low, rounded hills. 



Immediately south of Patterson Pass, resting unconformably upon the 

 granites, and rising abruptly above them, occur the quartzite formations. 

 They occupy the entire distance as far as the southern slopes of Pilot Peak, 

 the mass of that mountain being formed almost completely of brilliant 

 white quartzite beds. All the beds would appear to have undergone con- 

 siderable lateral compression, many of them standing at high angles, 

 producing a complicated structure, with local displacements ; the main 

 crumpling having produced both a synclinal and an anticlinal fold, with tlieir 

 axes striking diagonally across the topographical axis of the range. At 

 Patterson Pass, the quartzites strike north 30° east, with a steep dip to the 

 southeast, but shallowing southward as far as a low break in the range. 



