GEOLOGICAL STRUCTURE — EASTERN RANGE. 511 



crysts of augite, olivine and feldspar. Between these flows a zone of 

 decomposition several feet in thickness, colored brilliant red by peroxi- 

 dation of the iron, makes a prominent line, visible from a great distance, 

 on the bluff faces which almost completely surround the mesa. 



The surface of the mesa has a gentle slope westward and ends to the 

 eastward in an almost perpendicular escarpment overlooking the Tule 

 arroyo 1,500 feet below, which has here widened out into a considerable 

 valley that drains the northern slopes of the White range far to the 

 south. Beyond this valley, partly cutting off the view of the gulf of 

 California, lies the group of dark rugged peaks of metamorphic slates, 

 called the Volcan Peak group, which the Tule ar.oyo almost completely 

 encircles in its circuitous course to the sea. Through the gaps in this 

 range can be distinguished the pale blue waters of the gulf of California, 

 and occasionally portions of the coast-line, as well as several of the group 

 of small islands which lie a few miles off the shore in this latitude and 

 whose abrupt outlines show them to be probably projecting points of 

 the buried metamorphic ranges. 



The arms of the interior valley which lie to the south and west of the 

 Bluff Point mesa have a floor of granite which is entirely denuded of 

 the mesa sandstone covering and of the lake beds, if the latter ever 

 covered it. The granite is a light gray rock of normal type, consisting 

 of two feldspars, quartz and both white and brown mica. Hornblende 

 was not observed. From general appearance and association it would 

 appear to be a distinct and older rock than that found in the western 

 range. To the south of these valleys the White range, composed of the 

 same granite, stretches some 10 or 15 miles east and west across the 

 peninsula and apparently cuts off in great measure the interior valley in 

 this direction. As no contacts were found it was impossible to determine 

 the relative age of the granite and the metamorphic series. 



Circumstances- rendered it impossible to visit the Gulf coast, in spite 

 of its being probably less than fifteen miles distant in a direct line, but 

 the different distinct views obtained of it showed that a strip of mesa 

 land most everywhere separates the foothills of the buried ranges from it, 

 which would seem to be parts of the same series of deposits that have 

 been designated the mesa sandstones. 



No direct evidence was obtained either for or against the hypotliesis 

 that the Chico and Tejon beds once stretched entirely across the peninsula. 

 Nothing that could Ije lithologically identified with them was detected 

 in the region examined, and no organic remains whatever were observed 

 in what were called the mesa sandstones ; still, the negative evidence, 

 combined with the impression derived from the general structure of the 

 region, has given to the writer a strong feeling that these earlier beds 

 were confined to the western coast belt. 



