Review of Recent Geoloyiral Literature. 319 
Potomac: and the two higher members are called the Newer Potomac. 
'•Upon the whole," writes Prof . Ward, "I am disposed to consider the 
Potomac formation as a Cretaceous deposit, but as occupying nearly the 
whole interval from the close of the Jurassic to the base of the Upper 
Cretaceous, as that is commonly understood. The complete distribu- 
tion of its fossil plants will, I doubt not, justify this conclusion." 
It is now ascertained, as made known quite fully in this memoir, that 
the Raritan and Amboy clays of New Jersey belong to the same epoch 
as the Newer Potomac. Vjeing somewhat earlier than the Dakota forma- 
tion. 
The total Potomac flora comprises 737 distinct forms, consisting 
chiefly of ferns, cycads, conifers, and dicotyledons, the last named class 
being represented by 92 genera and 330 species. w. u. 
Sketch of the Geology of the San Francisco Peninsula. By Andrew 
C. Lawson. (Fifteenth U. S. Geol. Report, pp. .399-476, with eight 
plates, and three figures in the text.) The seven terranes of this area, 
in ascending order, are (1) crystalline limestone, of unknown age: (2) 
the Montara granite, intrusive in the preceding: (3) the Franciscan se- 
ries, of Mesozoic age, resting on the eroded Montara granite, an assem- 
blage of sedimentary and volcanic rocks of great thickness, with which 
are associated various basic intrusives, notably peridotite serpentines; 
(4) a light-colored, cavernous-weathering sandstone, supposed doubt- 
fully to be of Tejon age; (5) the Monterey series, of Miocene age, chiefly 
white, siliceous shale, resting on the last: (6) the Merced series, of Plio- 
cene age, a thick volume of sediments, with one stratum of volcanic 
ash, deposited after the erosion of the Monterey terrane; and (7) the 
terrace formations, of Pleistocene and Recent age, deposited after the 
disturbance and partial denudation of the Merced series. Among these, 
the Montara granite, the Franciscan series, and the Merced series, are 
the dominant features in the geology of the peninsula. 
The development of the present physiography is found, by Dr. Law- 
son's investigations, to have depended on the following principal geolog- 
ical factors: (1) the formation, one after the other, of two similarly tilted 
fault blocks sloping to the northeast and precipitous to the southwest; 
(2) the X)resence of subordinate faults and folds in these fault blocks; 
(3) the variously resistant character of the formations constituting these 
blocks: (4) the existence of the earlier and more northern block above 
l)aselevel and the consequent inauguration of its sculpture before the 
second block was thrown iip; (5) ensuing subsidence of the region and 
formation of the second fault block: (6) the emergence in unison of 
these two great block forms from the Pacific: (7) their sculpture as con- 
ditioned by (a) their relation to baselevel at various stages of the uplift, 
(h) their petrographic heterogeneity, and (c) their structural complexity: 
(8) the formation, as an episode in the uplift, of marine terrace deposits 
on the lower slopes of the fault V)locks: (9) the sculjjture of thepe ter- 
races; (10) associated accuiuulation of dunes; (11) a recent subsidence; 
(12) the formation of the present tidal flats, deltas, etc.: and (13) mod- 
ern shore erosion. w. v. 
