14 [APPENDIX 



are mostly a sandstone, which belongs to a similar range occurring on 

 the opposite shore. On the western flank of this portion of these moun 

 tains, or on the east shores of the bay San Pablo, and north part of San 

 Francisco, a range of sharp Peaks are seen ; their slopes are abrupt from 

 the summit for a considerable way down toward their base, and in the 

 latter part of winter and through the spring are covered with wild oats 

 and grasses. 



For several miles these mountains are made up of trapean rocks, 

 'which have forced themselves through the sedimentary rocks, and are in 

 all respects similar to the volcanic series that has protruded through the 

 older and also more recent formations in the Sierra Nevada. These rocks 

 are cut through and disturbed by a suite of igneous rocks of still more 

 recent origin, causing material change in the structure of the rocks that 

 preceded them, both sedimentary and plutonic. Among the latter we 

 find the pitchstone and obsidian, showing conclusively that they belong 

 to that class known as recent volcanic rocks. These rocks may be con 

 veniently observed in some of the lower hills about two miles east of the 

 house of Victor Castro, in the County of Alameda. The more recent 

 volcanic rocks form the first summit of the high ridge east of the bay, 

 and the frequently isolated conical peaks to which they give rise, and 

 which are known among the native inhabitants as (Picaches) stretch to 

 the north for six miles, when they are replaced by the older trap 

 rocks, and followed by the softer sandstones, which form the smooth 

 rounded summits of the hills to the Bay San Pablo and Straits of Car- 

 quinez. 



The sandstones on the shores present many marks of disturbance and 

 change, by the intrusion of the late igneous rocks, and it is not unfre- 

 quent to find fine threads of quartz ramifying through them in every 

 direction; when this is the case, or when they are in contact with 

 the more recent of the igneous group, their former structure is usu 

 ally found materially changed, a conchoidal fracture in these rocks is 

 often the result of this contact either in large or small masses. 



There are considerable areas of the sandstone in these mountains, that 

 present little or no traces of change, subsequent to their deposition, ex 

 cept that of uplift thus proving that they must have been deposited on 

 the trap rocks after the latter had become firm, and parted with their 

 heat : but where the sedimentary rocks are found in close proximity 

 with the later igneous rocks the change indeed in them is clear and de 

 cisive : thus demonstrating the fact that volcanic action has occurred at 

 a date long subsequent to the period in which these rocks were deposited, 

 and probably after their emergence above the surface of the sea, in which 

 they had their origin. It was found, in examining the sandstones lying 

 upon the older trapean rocks of this part of the range, that the fine 

 threads of quartz before noticed, gradually increased in dimensions as 

 they approximated the inferior strata of the sedimentary rocks, and 

 where the igneous rocks were exposed to view beneath them, in favor 

 able situations, they often exhibited true well and defined, though small 

 veins of quartz passing through them. 



These general characteristics continue south untill a point nearly east 

 of the town of Oakland is obtained, and here the trapean group of 

 rocks are interrupted to a certain extent. East of this town we find the 



