GEOLOGICAL STRUCTURE — INTERIOR VALLEY. 505 



diorites surrounding it. Along the center of the valley a low ridge of 

 rounded blocks of this very massive rock lias the appearance at first 

 glance of a morainal ridge with huge erratics, but examination shows 

 that the rounded forms are merely the result of weathering under the 

 peculiar climatic conditions of the region. The granite is an even grained, 

 granular rock, thickly studded with small black scales of mica and small 

 hornblendes. In thin section it shows a hypidiomorphic granular aggre- 

 gation of quartz, feldspars, black mica and deep green hornblende, with a 

 sprinkling of iron ores, apatite and rarely zircons. It resembles the 

 granites of the Sierra Nevada. 



Westward along the line of the section this granite is succeeded by the 

 finer-grained quartz-mica-diorite described above, and then by a belt 

 several miles in width of recent eruptives, which form low, rounded hills 

 adjoining the mesa region. These appeared to be mostly rhyolites, and 

 to have cut through the.sedimentaries of the mesa region, though it was 

 not possi])le to obtain unquestionable evidence of the latter fact. 



On the east, or at the liead of tlie Santa Caterina valley, capping the 

 flat ridges which form the western divide of the interior valleys, was 

 found a rather remarkable rock of the h vpersthene-andesite type, show- 

 ing microscopically small olivines and white feldspars, with occasional 

 black hornblendes in a dark gray matrix. In thin section it shows a 

 decided andesitic groundniass of augite and plagioclase microlites, Avith 

 the usual iron ores and abundant colorless olivines, pale hypersthenes, 

 small pale green augites and an occasional dark basaltic hornblende 

 with l)lack border. 



Abundant chalcedony and flint concretions with dendritic markings, 

 constituting the popularly known moss-agates, are found on the eastern 

 slopes of these ridges. 



Interior Valley. — The interior valley which was visited by the writers 

 is probably that designated by Gabb as the plain of Buena Vista. In 

 about latitude 30°, or a little north of the line of the section, its width 

 on a northeast-southwest line, or at right angles to the trend of the pe- 

 ninsula, is over fifteen miles, an almost level plain with a slight rise toward 

 its eastern rim, whicli rests on the submerged flanks and crests of* the 

 eastern range. Its elevation varies from about 2,000 feet on the western to 

 2,300 feet on the eastern edge. To the southward its widtli is contracted 

 by the encroachments of the bounding ranges which send out spurs or 

 ridges into it, and in the far distance appear to merge together. The 

 spurs from the eastern range are flat topped in great part and composed of 

 horizontally bedded material, which, where examined, consisted largely 

 of volcanic ash carrying abundant fragments of basic eruptives. These 

 are evidently the mesa sandstones of Gabb. Those from the western 



LXX-Bui,L. Geol. Soc Am., Vot,. f), 189.'1 



