186 Geology of the Bosphorus. 
found scattered through the gravel (which is about five feet 
deep), and upon the bed-rock. Occasionally they are found in 
the gravel and upon the bed-rock in the gulches from forty to 
sixty feet below the surface, but it is very rare in such locali- 
ties. It is most abundant upon the Eldorado bar situated on 
the Missouri River about sixteen miles from Helena; one man 
could collect on this bar from one to two pounds per day. 
I have had some of the stones cut, and among them one 
very perfect stone of three and a half carats, and of good green 
color, almost equal to the best oriental emerald. 
My opinion is that this locality is a far more reliable source 
for the gem variety of corundum than any other in the United 
States I have yet examined. 
Art. XX1.—The Geology of the Bosphorus. 
of the rocks of the Bosphorus, made in company with Mr. W. 
Forbes, Instructor in Mathematics in Robert College.—GEORGE 
WasHeBourn. | 
[Tux following notes are the result of a careful inane 
he B A 
THE straits of the Bosphorus conduct the waters of the Black 
Sea into the Sea of Marmora. The general direction of the 
stream is from N.E. to SW. Its length is about eighteen 
miles and its average width about one mile. Its depth is about 
the same as the height of its banks, which rise abruptly from 
its shores from two hundred to six hundred feet. These banks 
are broken by narrow valleys and steep gorges at right angles 
to the course of the Bosphorus. This broken, hilly country 
extends inland for many miles on both sides of the straits. 
The general character of the rocks is the same on both sides, 
but changes suddenly near Kavak, about six miles from the 
Black Sea, and again in the midst of old Stamboul at the mouth 
of the Marmora. The bleak and barren rocks above Kava 
are such as might be seen if a passage were cloven through Mt. 
Vesuvius, and this volcanic region extends, with irregular 
boundaries, about six miles east and about fourteen miles west 
of the Bosphorus. Below Kavak, as far as the Sea of Marmor, 
there are stratified rocks of the Paleozoic age, which extend 10- 
land about sixteen miles on the European shore, and about 
twenty-five miles on the Asiatic. They are bounded on the 
former by Tertiary strata, and on the latter by Cretaceous. 
_ Lhe Paleozoie rocks.—These rocks were formerly called Silu- 
rian. M. Roemer afterward claimed that they were ize Ue 
vonian. Tchihatcheff in his work on Asia Minor, Dr. bdullah 
Bey in various articles in German scientific journals, and Mr. 
