1889. | NEW YORK ACADEMY OF SCIENCES. dt 
shore of the ancient ocean, such differences of thickness may 
not be difficult of explanation. 
The whole region, like that of New York, is a most difficult 
one. The rocks are often decomposed to a great depth. Still, 
with care, certain belts can be traced with almost absolute cer- 
tainty for miles, but the contacts are concealed in a most ex- 
asperating manner. In Cream Valley there are numerous out- 
crops, but very poor exposures, and while I believe we know all 
the rocks of the valley, we do not know with certainty even 
their order of occurrence. The Potsdam is probably between the 
serpentine and the steatite, but this is not established. The 
dips are misleading and untrustworthy. 
Dr. NEWBERRY inquired about the length of the section, and 
as to the grounds on which the Laurentian and Potsdam had 
been identified; also as to the thickness of the latter. 
Mr. RAND replied that the section-scale was four inches to 
the mile, save in the portions immediately adjacent to the Lau- 
rentian, where it had been exaggerated (as shown on the Plate) 
in order to exhibit the various strata occurring within a narrow 
belt of a quarter of a mile. The total length across the crystal- 
line belt, from the ‘Triassic to the Cretaceous, is about twenty- 
five miles. The section-line approximates the course of the 
Pennsylvania Railroad, until the latter turns toward the S. W. 
along the northerly side of the South Valley Hill. 
As to the identification, the Laurentian has been deter- 
mined further to the N. W. Its characteristic mineral is an 
abundant blue quartz. As soon asthe area of these rocks is 
entered upon, the fact is indicated to the eye by the fragments 
of this bluish quartz in and upon the soil. The rock is further 
characterized by containing magnetite and zircon. 
The Potsdam is found along the same line as here indicated, 
further to the north, where it contains Scolithus. This fossil 
has not been found in Delaware Co.; but the peculiar rock—a 
feldspathic sandstone with rhomboidal fracture and micaceous 
partings, and containing numerous, though usually minute, 
tourmalines—is almost identical. Its thickness cannot be ac- 
curately given; it is between ten and fifty feet. 
Dr. NEWBERRY inquired further as to the presence of lime- 
stones or dolomites, so abundant in the rocks of the New York 
and Westchester region. 
