Vol. 4] Osmont. — Geological Section of Coast Ranges. 



65 



Valley it thins out very much. The great bodies above mentioned 

 at Franz Valley and near Twin Peaks seem to be of the nature 

 of agglomerate rather than tuff, containing an immense number 

 of angular fragments of lava, mostly andesite. At the latter place 

 fragments are sometimes encountered many tons in weight. 



The Interbedded Sandstones and Conglomerates. — Toward 

 the west, on both sides of Santa Rosa Valley, beds of the pumi- 

 cious tuff, ten or fifteen feet thick, are interbedded with sand- 

 stone and gravel. The sandstone is usually a rather coarse, yel- 

 low, loosely compacted rock. The conglomerate is very distinc- 

 tive. It is made up almost entirely of volcanies. The pebbles 

 are mostly andesite, some appear to lie basalt, and quite a large 

 proportion is a white rhyolite which the writer has never encoun- 

 tered in place. Rhyolitic breccias, and occasional pebbles of 

 Franciscan chert, glassy obsidian and petrified wood, also occur. 

 Between Mark West Springs and the edge of the Santa Rosa 

 Valley, along the line of Section AB, these gravels are very 

 coarse, 75% being over two inches in diameter, and 10% over 

 six inches. At this locality fully 1,900 feet of these buff colored 

 sandstones and heavy gravels rest on top of about 200 feet of the 

 pumicious tuff, but on the east limb of the anticline, and in the 

 synclinal basin of Mark West Creek east of the springs, the same 

 character of gravel is seen to be interbedded with considerable 

 thicknesses of tuff, though at the base is nearly 600 feet of tuff 

 free from sandstone and gravel. Thus the lower portion of the 

 beds at this point are wholly tuffaeeous, while the upper part 

 contains no tuff at all, though the gravel is the same. On the 

 east side of the anticline there is no line of demarcation between 

 the tuff and the sandstone. Tuff seems to be interbedded with 

 sandstone up to a considerable distance above the base, and then 

 sandstone continues to the top. On Section AB two beds have 

 been shown, one representing the tuff and the other the sand- 

 stone, but the line between the two is an arbitrary one, as the 

 writer could not find it in the field, and is of the opinion that 

 tuffs and sandstones belong to the same period of sedimentation. 



Lava Flows within the Sonoma Tuff- -Interca Hated with the 

 tuff, on the east side of the anticline at Mai'k West Springs, about 

 300 feet from the base, is a forty foot flow of basalt. At Gree- 



