GEOLOGY OF THE COAST SYSTEM OF MOUNTAINS. a 
(3) Sandstones aggregating 2,000 feet in thickness. 
(4) A formation of radiolarian cherts from 100 to 900 feet. 
(5) Sandstone, 1,000 feet. 
(6) Radiolarian cherts, 500 feet. 
(7) Sandstone, 1,400 feet. 
In this sequence of sedimentary strata, particularly toward its upper part, there are 
intercalated lavas at various horizons. 
After their accumulation, but before the next higher series of rocks was deposited upon 
them, the Franciscan strata were invaded by intrusive rocks at points so numerous and 
so widespread thruout the Coast Ranges that these intrusive bodies constitute one 
of their most characteristic associations, in contrast to the series which succeed them. 
The intrusive rocks are of two general types. One is a highly magnesian rock, usually 
a peridotite, but with facies of pyroxenite and gabbro, the peridotite being generally 
almost completely serpentinized. The other is a basaltic rock grading into diabase and 
having in many of its occurrences the peculiar structure characteristic of the spheroidal 
basalts. In addition to the spheroidal structure on the gross scale, it is in some cases 
variolitic. Associated with both of these intrusives are areas, generally of limited 
extent and sporadic distribution, of glaucophane and other crystalline schists, which 
appear, where they have been most thoroly studied, to be the result of a peculiar kind of 
contact metamorphism. 
The stratigraphic composition of the Franciscan series indicates an interesting to-and- 
fro migration of the shore line of that time, probably due to a vertical oscillation of the 
continental margin. The basal group of sandstones, shales, and conglomerates is clearly 
a terrigenous deposit laid down in proximity to the margin of the continental area from 
which the sediments were derived. The next succeeding formation, the foraminiferal 
limestone, on the contrary, is nonterrigenous. Its character as nearly pure carbonate 
of lime, except for the flinty lenses and nodules it contains, and the abundance of foram- 
inifera, indicates that the sea-bottom over the present position of the San Francisco 
Peninsula was too remote from the shore to receive an admixture of sand or clay. That 
is to say, the conditions which favored the deposition of the limestone were inaugurated 
by a withdrawal of the shore line from the position which it occupied during the deposi- 
tion of the underlying sandstones. And this lateral migration of the shore was doubtless 
the result of a sinking of the coast. 
Above the foraminiferal limestone sandstones again occur, indicating a return of the 
shore to about its former position, doubtless due to an uplift of the sea-bottom and coast. 
These sandstones are in turn followed by a nonterrigenous formation of radiolarian cherts. 
These are for the most part flinty rocks containing abundant remains of radiolaria, 
marine organisms which secrete a siliceous test instead of a calcareous one, as in the 
ease of the foraminifera. They contain no admixture of sand, and the shaly partings 
which separate the layers of chert are very doubtfully referable to land waste. Here 
again the sea bottom must have been deprest and the shore line caused to withdraw. 
These radiolarian cherts are followed again by sandstones, and these by a second forma- 
tion of radiolarian cherts, the former as before indicating uplift of the sea-bottom and the 
latter depression. The last movement in Franciscan time was uplift, indicated by the 
sandstones, which rest upon the second horizon of radiolarian cherts and which constitute 
the topmost formation of the Franciscan series. 
The age of the Franciscan is not positively known. Certain general considerations, 
however, contribute data upon which a tentative judgment as to this question may be 
based. Stratigraphically, the Franciscan lies upon the eroded surface of the Coast Range 
granites, the correlation of which with the post-Jurassic granites of the Sierra Nevada 
has been suggested. If such correlation be adopted, the age of the Franciscan must be 
