16 THE POECUPINE PLACER DISTRICT, ALASKA. [bull. 23d 
tion only to Salmon River, where it disappears. Porcupine and other 
gold-hearing creeks of this district lie mainly in the sedimentary rocks, 
though their upper portions cut into the diorite band to the south. 
This band has been the source of the large bowlders contained in the 
creek wash (PI. VI). 
SEDIMENTARIES. 
The sedimentary rocks, as before stated, consist of a series of slates! 
with interbedded limestone. The slates are mostly black; some of 
them soft and rich in graphite, othei\s hard and flinty, while adjacent 
to the limestone beds they are usually more or less calcareous. The 
limestones are sometimes fossil bearing, but as a rule they show evi- 
dence of considerable dynamic action, which has contorted and pressed 
the shells so that it is difficult to recognize their original form. The 
general strike of the sedimentaries is N. 60° W., dipping 75° NE., 
though this varies greatly. The beds are often wrinkled and folded, 
but apparently on a small scale, as no repetition of groups of strata 
was observed; The diorite contact usually follows the stratification,, 
though in some places the intrusive masses crosscut the slates. Next 
to the diorite the slate has been baked and altered to a flinty horn-] 
stone for a Avidth of several hundred feet. Similarly the limestone 
beds have also suffered alteration to white tine-grained crystalline 
marble. A good example of this is shown just back of Pleasant Camp 
(See map, PI. V). 
A small collection of fossils gathered from a stratum of limestone 
on Porcupine Creek were determined to be of lower Carboniferous 
age by Dr. G. H. Girty, of the United States Geological Survey. The 
following is an abstract from his report: 
The forms identified are: 
Crinoidal fragments. Spirifer striatus. 
Productus latissinms. Camarophoria ? sp. 
Productus semireticulatus. 
The age is clearly Carboniferous, and though I hesitate to offer a further opinion 
upon a fauna which is more or less strange, I believe that it is of lower Carboniferous 
age. The faunal type, however, is that of the lower Carboniferous of the west coast 
and not that of the Mississippi Valley and Eastern States. I believe that it is to be j 
correlated with the later portion of the lower Carboniferous of the Mississippi Valley, 
or even that it is somewhat younger. Lower Carboniferous horizons have already 
been reported from Alaska, but the present fauna is not precisely the same as any 
of those so far discovered. 
In a report on the Ketchikan mining district Mr. Brooks a calls atten- 
tion to a coral found in the debris of Dirt Glacier of Glacier Bay, 
which was determined by Mr. Schuchert as belonging to the Car- 
boniferous age. This district, however, is 40 miles south of Porcu- 
a Brooks, Alfred H., Preliminary report on the Ketchikan mining district, Alaska: Prof. Paper 
U. S. Geol. Survey No. 1, 1902. 
