38 [APPENDIX 



had their intrusion corresponded with those of a similar character found 

 among the sedimentary rocks on every side of them. Our evidence that 

 these rocks hold an age anterior to the tertian es, rests in part on this 

 fact ; that if the change of position, Avhich the latter rocks have suffered 

 in adjoining districts bear any testimony of the maximum of .disturbance 

 among them, we should find some corresponding changes of similar cha 

 racter had this volcanic group been of the same age. 



In all other parts of the coast mountains, wherever volcanic rocks 

 occur, in masses or dikes, cutting either sedimentary or primitive groups, 

 the evidences of uplift are conspicuous, and either a high uniform grade, 

 distinct undulations, or a terraced form of the valley sections, mark the 

 extent and character of the forces exerted over such areas ; while in the 

 case before us, we find the valley adjoining these igneous outbursts main- 

 taininglts level to the very base of these volcanic hills. Were there 

 any differences observable in the inclination of the surface of the plain, 

 taken from its northern to its extreme southern point, there would be 

 some reason to suppose that the group under consideration was more 

 recent than the tertiary era. 



Passing to the west of these hills, we find the same undisturbed con 

 dition of the surface, extending to the base of the Santa Cruz, and you 

 meet with nothing that would in the least indicate any subsequent alte 

 ration in the general level until you reach their eastern base ; and here, 

 for the first time, a narrow terrace is found, its face rising rather abruptly 

 for tw elve feet, and then very gradually for eight feet more, being nearly 

 level from thence to the mountains, a distance of one-fourth of a mile. So 

 little disturbance is manifest in the half formed conglomerate, of which 

 it is formed, that it must be looked upon as having been gradually ele 

 vated by forces that have acted equally, and at the same time, on both 

 the older igneous, and recent formations of this section, and the forces 

 that thus gradually operated in producing these phenomena are unques 

 tionably persistent at the present time. 



MOST RECENT VOLCANIC ROCKS OF THE COAST 

 MOUNTAINS. 



Under this head, a brief description of a class of rocks differing essen 

 tially in relative position, lithological character, and general effects on 

 the surrounding country in which they are situated, from those noticed 

 in the preceding chapter, will be given. The section of the country 

 though which they may be observed, has been stated to extend through 

 about forty miles in length of the Monte Diablo Range, and make their 

 appearance in the Santa Anna Pi caches, on the south-east border of the 

 Santa Clara valley. 



In examining this group it was found, that they consisted principally 

 of compact and vessicular lavas, having different degrees of firmness 

 and texture. 



A conspicuous and striking feature of these rocks is found in their 

 connection with other formations, both igneous and sedimentary, and in 

 the case of the latter class they are seen to have acted directly upon 

 them, elevating the same to different altitudes above the sea level The 



