74 



University of California. 



[Vol. i. 



exposed to afford observations for dip and strike, would seem to 

 underlie the jaspers, and to belong to a lower horizon than the sand- 

 stone previously referred to. 



After the area had had substantially its present topographical 

 features impressed upon it, it was submerged, and covered with a 

 deposit, of which vestiges remain up to the summit of the hill upon 

 which the old tower stands, now 280 feet above the sea. This 

 deposit is most conspicuously shown just south of the stream 

 mouth, at A, where it forms a nearly vertical bluff, more than 100 

 feet in height, of horizontally stratified material, only partially con- 

 solidated. At the base of the bluff this consists of pebbles of 

 volcanic rock and jasper, while the highest portions of the formation 

 are composed of a clayey sand, which readily disintegrates into a 

 loose sandy loam. 



It has been said that a portion of the eruptive rock is intrusive 

 into the overlying sedimentary formations; with this rock, however, 

 are associated other rocks, which are certainly not intrusive, but 

 which possess, on the contrary, all the features which are generally 

 accepted as characterizing lavas extravasated upon the surface. 

 The most typical facies of the two rocks may be readily distin- 

 guished in the field, the one being compact and amygdaloidal, 

 showing no crystals to the unaided eye, and possessing a very 

 marked spheroidal structure, the other being a distinctly crystalline 

 rock of almost gabbroitic aspect, traversed by irregular joint planes, 

 and without spheroidal structure. The latter rock does not, how- 

 ever, always preserve its distinctive character, and may be traced, 

 both in the field and in the microscopic slides, into modifications 

 not distinguishable with absolute certainty from the typical sphe- 

 roidal rock. The chemical analyses show that both rocks are prob- 

 ably derived from the same basic magma. 



The problem thus presented for solution requires that the erup- 

 tive terrane be differentiated throughout into the two formations 

 which are clearly shown in certain places; that the relations of the 

 two divisions to each other, and to the sedimentary series, be 

 unraveled; and that some explanation be offered for the various 

 structural peculiarities of the eruptive rocks presently to be 

 described. 



