SAN FRANCISCO SANDSTONE—LITHOLOGICAL CHARACTERS. 
147 
crystalline rocks; these are nearly vertical to the planes of stratification, and cut through all 
the beds. Seams are thus formed for the access of air and water. 
The color of this sandstone is dark bluish-green, inclining to gray. It is exceedingly compact 
and tough, and does not break so easily as the fine-grained red sandstone of the Connecticut 
river and New Jersey quarries. Its texture varies but little in the different beds ; the grain is 
close and even, and generally very fine. 
The grains are chiefly silicious, and are mingled with those of finely triturated glassy feldspar 
and other minerals, the whole being apparently cemented together by fine particles, or probably, 
in part, by carbonate of lime. Nearly all of the specimens submitted to examination were found 
to be calcareous, and effervesced freely with acid. This is true of specimens from the other 
localities, so far as examined, and the rock may with propriety be called a calcareous sandstone. 
Oxide of iron is an important constituent of the rock, and it is probably in the form of protoxide, 
for the interior portions are greenish-blue, and on exposure become rusty brown or drab, the 
result of the formation of the sesquioxide. The iron may be present in combination with sulphur; 
one or two minute grains of pyrites were detected in some of the specimens. 
Numerous films of a dark slate-colored substance are distributed in parallel planes through 
the mass of the rock. They are often coal black, and some are soft, like clay or shale, and are 
sometimes in small lenticular masses, which can be easily excavated with the point of a knife. 
Some of the beds contain many more of these masses than others, and they determine the direc¬ 
tion of easy fracture of the rock. One of the beds of soft shale was highly charged with black 
masses, and they were, without doubt, the remains of plants, a coaly substance being distinct in 
the thickest layers. The films in other parts of the rock were, in most cases, composed chiefly 
of clay, and may be fragments of shale. 
The position of these beds of sandstone is highly favorable to the operation of quarrying, and 
the stone can be readily loaded at a wharf and ferried over the channel to the city. It will be 
found to be a highly valuable and elegant building material. 
The same formation of sandstone and shales underlies the city, but there is no exposure where 
the strata can be so conveniently studied as at Yerba Buena, or where they are so free from decom¬ 
position and fissures. The strata are apparently much more bent and broken up, and the action of 
the atmosphere and surface water has extended to a greater depth below the surface. The best 
section, at the time of my observations, was along Pacific street, where Telegraph Hill had been 
cut away. At that place the stratification was very distinct, and the alternation of thick beds 
of argillaceous sandstone, with shales and slate, was visible. Numerous curves and flexures of 
the beds render the dip variable, but its prevailing direction is eastward. The rocks crop out 
on the top of the hill, and look like ordinary trap-rock which has been exposed to the weather. 
These rocks are again exposed along Dupont street and Broadway, and they form the shores of 
North Point. They also project out into the channel between Telegraph Hill and Fort Point, 
forming Tonquin Point and Point San Josef. At all these places decomposition of the rock 
has extended so deeply that the unaltered portions had not been reacked by the excavations, and 
the true color and lithological characters of the rock were not exhibited. At one or two points 
about the city, however, deep excavations into the rock for wells showed that, in color and com¬ 
position, the rock was similar to that at Yerba Buena. In fact, the general characters of the 
strata are the same, and the rusted and decomposed exterior crust of rock has nearly the same 
appearance at both places. The detailed description of the Yerba Buena stone may, therefore. 
