350 'The American Geologist. June, 1895 
stratigraphical and geographical distribution of these pro- 
foundly interesting rocks, as well as of their petrographical 
character. It suggests farther an explanation of the very 
remarkable bedding in the supposition that the supply of 
silica may have been rhythmically intermittant. We have as 
yet no measure of the rate of their accumulation. 
Volcanic Hocks. — It would be out of place to attempt here 
a detailed petrographical description of the volcanic constit- 
uents of the Franciscan series. They are, in general, basaltic 
rocks in a more or less advanced state of alteration. Some- 
times they are dense compact lavas, sometimes they are 
highly ani^^gdaloidal. Occasionally they have the character 
of glass breccias, and pyroclastic accumulations are abun- 
dantly represented. These effusive rocks appear interstrati- 
fied with the sedimentary strata at various horizons in beds 
varying in thickness from a few feet to several hundred feet. 
Intrusive Jlocks. — The igneous rocks intrusive in the Fran- 
ciscan series may be classed under two heads. 1. Those of 
a diabasic or basaltic character, and (2) peridotite (some- 
times p3n-oxenite) now serpentinized. Of the first class many 
are identical with the spheroidal and variolitic basalts of 
point Bonita, which have been described by Ransome*. Others 
are of the character of diabase or olivine diabase. While 
many of the occurrences of these rocks are clearly intrusive, 
there are other cases where it is very difficult to discriminate 
them from the contemporaneous effusives, and it is probable 
that part of them are genetically connected with the latter 
and represent a phase of the same volcanic activity. There 
are also some intrusives of very limited extent of the charac- 
ter of gabbros or diorites and one occurrence of a rock which 
may prove on further examination to belong to the bostonites. 
The peridotites and pyroxenites, which, by well known 
process of alteration have given rise to the serpentines, form 
an important feature of the geology of the peninsula. There 
are two dominant ranges of the serpentine traversing the 
peninsula from northwest to southeast; and there are other 
subordinate occurrences having a similar trend. The field 
study of these serpentines shows that beyond any question 
they are of the nature of dikes, intrusive sheets and laccolitic 
*Bull. Dept. Geol. Univ. Gal., vol. i, no. .'5. 
