GEOLOGY AND NATURAL HISTORY OF LOWER CALIFORNIA. 979 



less than 4,000 feet above sea level. Between the peaks are broad, 

 transverse valleys and flat-topped ridges whose higher summits have 

 the same general level with tliose of the higher plateaus of the mesa 

 region — that is, about 2,000 feet. Eounded pebbles and an occasional 

 fragment of recent shells were found on these summits, which 

 strengthen the opinion that this was a peneplain of recent times, 

 probably formed at the time of the greatest submergence since the 

 deposition of the mesa sandstones. 



" The range was traversed on two lines — that of the arroyo of Santa 

 Caterina, shown in the section [Plate 1], and that of the Rosario arroyo. 

 The river bed or arroyo of San Fernando crosses it about midway 

 between these two. Near the mission of San Fernando is a consider- 

 able development of sedimentary beds, one of which is a much altered 

 bluish limestone containing unrecognizable fossils, which is probably 

 either of early Mesozoic or Paleozoic age. [Plate 3.J The beds have a 

 steep dip to the eastward; at one point are overturned against a con- 

 siderable body of acid eruptives and diorite. On the line of the Rosario 

 arroyo it consists mainly of diabase, with acid eruptives and diorites 

 on their eastern flank. The latter cut the diabases, and are succeeded 

 on the east by an extensive flow of rhyolite capping the mesa ridges 

 which extend out into the interior valley. A little farther south dio- 

 rites seem to form the main mass of the flat topped ridges which here 

 represent the range, and which are flanked on the east, at the border 

 of the mesa region, l>y recent tufaceous rocks, in which is found one of 

 the few springs of the region. Along the line of the section south of 

 San Fernando, diorites again predominate, and in these occur deposits 

 of copper sulphides, one of which has been quite extensively mined. 



"It was not possible to determine the relative age of all the varieties 

 of eruptive rock observed, but the older eruptives are evidently pre- 

 Cbico, while some of the recent eruptives are certainly more recent than 

 the mesa sandstones. 



" The rocks described above as acid eruptives are comjiact and some- 

 times brecciated quartz-porphyries of greenish and brownish colors, at 

 times quite aphanitic, and again showing small phenocrysts of feldspar 

 and more rarely quartz, suflQciently developed to be recognizable by the 

 naked eye. Chemical tests in the more aphanitic varieties yield 70 to 

 75 per cent silica. The more common form of the diorite is a pinkish 

 gray, finely granular rock, which in thin section shows a hypidiomorphic 

 granular aggregate of quartz and triclinic feldspar with pale green 

 hornblendes, in jjart or wholly altered to epidote. There are also a few 

 sphenes and the usual iron ores. 



" In the upper Santa Caterina Valley, which crosses the range diag- 

 onally in a nearly north-and-south direction, a very considerable mass 

 of underlying granitic rock is exposed over an extent of about 10 miles 

 along the bottom of the valley, which apparently grades into the finer- 

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

 ridge of rounded blocks of this very massive rock has the appearance 



