GEOLOGY OF CARRIZO MOUNTAIN, CALIFORNIA 347 



the deposition of the muds. The other hypothesis, namely, that 

 these beach sands and the muds were deposited contemporaneously, 

 because of differing local conditions, is made difficult to apply because 

 the two physically different terranes overlie the basement rocks at 

 very closely adjacent points, with no obvious explanation as to why 

 such different conditions should have prevailed so near together. 



The heavy sandstones occur at a number of places along the 

 north slope of Carrizo Mountain, east of the head of Garnet Canyon. 

 Many arroyos are incised in them, the stream channel in some cases 

 being a mere notch but a few feet wide and a hundred or more deep. 

 Fossils, however, have not been reported in numbers except at the 

 head of Garnet Canyon, 



At the head of Barrett Canyon, which drains south from Black 

 Mountain, the same general relations prevail that have been described 

 in the area a dozen miles to the south on the slopes of Carrizo Moun- 

 tain. But the fragments of the basal beds of the Miocene are even 

 more widely scattered, and the sandstones ,and conglomerates are not 

 so fully developed. 



About 4^ miles above the mouth and one-half mile above the 

 forks of the Arroyo, the basal beds of the Miocene flank the older 

 rocks and extend across the valley from the west fork to the east 

 fork. Dips here are 20 to 40 degrees to the south, i.e., away from 

 the mountain. The beds are not so thick as on Carrizo Mountain 

 but are succeeded, as is the case there, by soft yellow shales. 



About a mile above this point, in a little cove at the head of a 

 small western tributary of Barrett Creek, other outcrops of basal 

 sandstone and conglomerate, not more than 10 feet thick, occur with 

 the underlying igneous rocks all about them. Near this outcrop is 

 a fossil coral reef lying directly upon the lavas and isolated from all 

 the other sedimentaries (Fig. 6). A half-mile farther north a sheet 

 of sandstone, folded into a basin and thus somewhat protected from 

 erosion, still exists. Doubtless many other similar fragmental 

 exposures would be revealed by more extended search. 



Miocene shales. — ^Flanking Carrizo Mountain on nearly all sides 

 and extending on the southeast practically to Signal Mountain, on 

 the international boundary, are continuous exposures of the beds 

 which overlie the basal conglomerate. They are well developed also 



