1893. ] NEW YORK ACADEMY OF SCIENCES. 107 
which he collected. The writer takes this opportunity to ex- 
press his acknowledgments. 
DIscUssION. 
Prof. Kemp remarked on the general presence of this mineral 
containing the rare earths of the cerium group, along the white 
limestone belt of Sussex county, N. J.,and Orange county, N. 
Y., in association with granite intrusions. 
A Pleistocene Lake-Bed at Elizabethtown, Essex 
County, New York.* 
By Hernricu Riss. 
The drift deposited by the ice sheet in its passage over New 
York formed in many instances dams across the valleys, causing 
an interruption of the drainage, with the consequent formation 
of lakes. Some of these still remain, but the majority of them 
have been obliterated by sedimentation or the lowering of their 
outlets, so that at the present day Pleistocene lake-beds may be 
seen in many of the valleys of the State. They are very 
numerous in Southern New York and also in the Adirondacks. 
While engaged at field work in Essex county with Prof. Kemp 
last September, I was most forcibly impressed with the large 
number of these lakes of obstruction, as well as the beds of 
previously existing ones. Emmons [Geol. of N. Y., Part IV., 
p. 212] says: “There are about one hundred lakes in Essex 
county, most of which are small. They diversify the surface of 
the country, and impart a great variety to its scenery, but con- 
tribute considerably to diminish its temperature. They are not 
evenly distributed over the country, but are collected in small 
clusters about the different summit levels in different portions 
of the county. Most of them are small and narrow, and instead 
of occupying shallow basins scooped out of the softer materials 
as earths or ordinary slates and shales, they lie in chasms 
formed by the uplifts and fractures in the primary rocks.” 
On account of the narrowness of these fault valleys, which 
Emmons mentions, a comparative small amount of drift was 
necessary to form an obstruction across them, backing up the 
water of the stream and forming alake. If the lake thus formed 
is small and shallow, a section of its bed simply shows suc- 
cessive layers of sand or silt, but if it be of considerable size 
* Published by permission of Prof. James Hall, State Geologist. 
