980 REPORT OF NATIONAL MUSEUM, 1895. 



at first glance of a morainal ridge with liuge erratics, but examination 

 shows that the rounded forms are merely the results of weathering 

 under the peculiar climatic conditions ot 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 hypidio- 

 morphic granular aggregation 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 adjoing the mesa region. These appeared to be mostly rhyolites, 

 and to have cut through the sedimeutaries of the mesa region, though 

 it was not possible to obtain unquestionable evidence of the latter fact. 



"On the east, or at the head of the Santa Caterina Valley, capping 

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

 was found a rather remarkable rock of the hypersthene-audesite type, 

 showing microscopically small olivines and white feldspars, with occa- 

 sional black hornblendes in a dark gray matrix. In tliin section it 

 shows a decided andesitic ground mass of augiteand plagioclase micro- 

 lites, with the usual iron ores and abundant colorless olivines, pale 

 hypersthenes, small pale green augites, and an occasional dark basaltic 

 hornblende with black 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 peninsula, is over 

 15 miles, an almost level plain with a slight rise toward its eastern rim, 

 which 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 width 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 erup- 

 tives. These are evidently the mesa sandstones of Gabb. Those 

 from the western range, on the other hand, are composed largely of 

 eruptive rocks and appear to be projecting portions of the older moun- 

 tains laid bare by erosion, but in a few cases are mesa-topped ridges, 

 capped by horizontal lava flows of later age than the mesa sandstones. 



