202 
years a new ice sheet will irresistibly push its way as far south as 
its predecessors, deleting the work of man on its way and building 
a new terrain, on the surface of which, after that ice retreats, 
there may develop modified floras and faunas, new races of men, 
and new civilizations. 
COMPOSITION AND SOURCES OF THE BOTANIC 
GARDEN BOULDERS 
As already stated in this Guipr, one of the features of erratic 
1e fact 
— 
boulders that arrested the attention of carly observers was t 
that they were of different composition from the underlying bed- 
rock. Where the terrain was wholly of glacial origin, as in the 
case of most of Long Island, there was, of course, no bedrock, 
but in such cases it was noted that the boulders had the composi- 
tion of bedrock to the north. 
A technique has been perfected whereby sections of rock can 
be prepared so thin that they may be viewed under the microscope 
with transmitted light. Studies made in this way enable one to 
ascertain the composition of a rock sample, and to compare one 
sample with another, with great accuracy. Through the coopera- 
tion of Dr. Robert Balk, professor of geology in Hunter College, 
New York City, samples were taken from about thirty of the 
larger or otherwise more interesting boulders in the Botanic Gar- 
den and studied microscopically, as just described. Comparisons 
were then made with the already known composition of outcrops 
of bedrock north of Long Island. In this way it has been pos- 
sible to determine, with a fair degree of accuracy, the general loca- 
tion of the ledges of which our boulders were formerly a part and 
from which they were picked up by the southward moving conti- 
nental ice sheet, carried to the southern edge of the ice, and there 
deposited, to become ultimately a geological enigma, a source of 
troublesome bother in grading operations, a valuable feature of 
the landscape, building material for rock garden and bridges, and 
the subject of this Guipe and of innumerable other geological 
treatises. 
Through the generosity of Mr. Idward C. Blum, president of 
our Board of Trustees, a fund was provided for the casting of 
