504 EMMONS AND MERRILL — SKETCH OF LOWER CALIFORNIA. 



occasional fragment of recent shells were fonnd on these summits, which 

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

 ably 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 19), and that of the Rosario arroyo. 

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

 tween these two. Near the mission of San Fernando is a considerable 

 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. The beds have a steep dip to the east- 

 ward, at one point are overturned against a considerable 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 in- 

 terior valley. A little further south diorites 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, by recent tuffaceous 

 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-Chico, 

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

 mesa sandstones. 



The rocks described above as acid eruptives are compact 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, sufficiently developed to be recognizable by the 

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

 75 per cent silica. 



The most common form of the diorite is a pinkish gray finely granular 

 rock which in thin section shows a h3^pidiomorphic granular aggregate 

 of quartz and triclinic feldspar with pale green hornblendes in part 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 diagonally 

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

 lying granitic rock is exposed over an extent of about 10 miles along the 

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



