The Geoloyy of the Coast Ranges. — Lavson. 347 
This presumption must be kept steadil^^ in view till it is nega- 
tived by positive evidence. 
The rocks of the Franciscan series occupy two distinct areas, 
one on either side of Merced valley, which traverses the pen- 
insula obliquely from northwest to southeast. The southern 
area is a belt of from four to five miles wide which flanks 
Montara mountain on its northeast side and conforms to its 
strike. The belt extends from the steeper slopes of the moun- 
tain out to the edge of the Merced valley, and farther south. 
where the terrane is not now represented, out to the shores of 
the bay. The northern area of the Franciscan terrane lies in 
the triangular section of the peninsula which is situated be- 
tween Merced valley, the Golden Gate and the bay of San 
Francisco. The formations of the series, with the associated 
eruptives occupy all of the seemingly irregular cluster of 
peaks and ridges within the area indicated. Other rocks, 
however, share with the Franciscan series the occupancy of 
the area. The terrace formations and the sand dunes wind 
in among the hills and ridges, and cover their lower flanks. 
The various petrographically discrete formations which 
make up the Franciscan series are : 
1. A basal formation of conglomerates, coarse grits, sandstones, shaly 
sandstones, shales and argillaceous limestones. 
2. The "San Francisco sandstone," the dominant sedimentary form- 
ation of the series; a moderately fine grained sandstone, fairly uniform 
in character over large areas, with subordinate beds of shale and con- 
gU)merate. The sandstone is uniform not only in its lateral extension 
but also vertically for great thicknesses. It is interbedded with for- 
mations 3, 4 and 5 named below. 
8. P'oramini feral limestones. 
4. Radiolarian cherts. 
5. Volcanic rocks including basaltic lavas and pyroclastic accumula- 
tions. There are besides these, intrusive rocksof a corresponding cliar- 
acter. some of which are probably connected with these extravasations, 
and also intrusive peridotites and pyroxenites now serpentinized. 
6. Silica-carbonate sinter. 
In addition to these there are certain metamorphic schists 
which arise from the local alteration of the sedimentary or 
volcanic formations and do not constitute a separate forma- 
tion according to the writer's interpretation of them. 
Of these various kinds of rock those classed under 1 and 2 
