170 
GEOLOGY. 
of iron ore in strata of various ages. The origin of the gypsum, which abounds in the strata in 
thin transparent plates or fibrous masses, is also shown by the phenomena. There is little 
doubt that the gypsum crystallizes from the solutions of lime, which must accumulate in the 
strata below those in which oxide or carbonate of iron has been formed. The appearance of 
interstratification that is presented by the gypsum is probably due to the fact, that the solution, 
in its downward progress, meets with a retentive layer of clay or other materials, or that other 
conditions, favorable for crystallization, exist in some of the strata and not in others. The 
complete separation of the selenite from the earthy materials holding the solution is an inter¬ 
esting result of the action of the forces of crystallization, and must of necessity produce a con¬ 
densation of the strata above and below. It is known that a separation of this kind results when 
water freezes in a bank of earth, thin sheets of pure ice being formed in horizontal planes, and 
thus elevating or condensing the earth above them. Another evidence of the correctness of this 
explanation of the origin of layers of gypsum in stratified deposits is found in the fact that 
thin seams occur in nearly horizontal planes in some of the highly inclined sandstone strata of 
Navy Point, Benicia. They were not found in all of the beds, and the layers frequently end 
abruptly against the sides of the stratum in which they occur. It was evident that they were 
the result of infiltration. The great deposits of gypsum which are found along the Canadian 
and Red rivers and on the Pecos probably have a similar origin. . The sulphuric acid, in either 
instance, may have been supplied by springs, or by volcanic vents when the strata were sub¬ 
merged, or by the decomposition of other salts than sulphate of iron. 
The peculiarly contorted and waved lines found in the sandstone strata at Benicia, in beds 
which rested conformably with others in which the lines of lamination were perfectly straight, 
are, without doubt, produced by infiltration, and connected with the formation of the seams of 
gypsum. In the Tertiary of Virginia, along the lower part of the Potomac, it is known that 
fossils cannot be obtained where gypsum abounds. Casts of the shells are all that remain, the 
lime having doubtless been taken to form the gypsum. 
Fossils. —The bed of fossil shells, from which specimens were procured that have been the 
means of identifying this formation as Miocene, is near the spot we occupied as the Depot Camp, 
and on the left bank of the creek. It was a source of much regret not only that the shells had 
all been dissolved out, but that the extremely perfect impressions and casts which were left in 
the clay and sand were so friable that they could not be transported. A person skilled in con- 
chology could not fail to determine the species at once from these impressions, so perfect were 
they in every part, even to the finest lines of the former surface of the shell. A few specimens, 
which were more highly charged with oxide of iron than the others, were procured and trans¬ 
ported in safety; but multitudes of the most interesting forms could not be carried away without 
crumbling. Being without any glue with which to saturate these specimens, the only resource 
was to make careful drawings upon the spot, representing, as nearly as possible, all the impor¬ 
tant characters, so that the species might be determined from them. Selections from the draw¬ 
ings thus made will be found on Plates III to IX. They were forwarded to Mr. Conrad, of 
Philadelphia, together with some of the specimens, and his report upon them will be found in 
full in the Appendix. Mr. Conrad considers all these fossils as new species, and has named 
them as follows: Natica Ocoyana, N. geniculata, Bulla jugularis, Pleurotoma transmontana, 
P. Ocoyana, Sycotopus Ocoyanus, Turrltella Ocoyana, Coins arctatus , Tellina Ocoyana, Pecten 
Nevadanus, P. catillijornis. Casts of the following genera were identified, but the species 
could not be determined : Cardium, Area, Solen, Dosinia, Venus, Cytherea. 
