258 The American Geologist. ^p"""' i^^^s 
lake and 65 feet below it. The grit is prol^ably about 230 
feet thick here. The walls are pierced by four crevasses 
now filled with drift — the remains of two fissures crossing 
each other at the deepest point in the lake, 74 feet deep. 
There is no drift in the lake basin, not even under the south- 
facing cliffs, although the fissure running S. 25° W. is filled, 
and the transverse breach is blocked to 150 feet above the 
lake. The glaciation is here S. 10° W. The cause of the 
absence of drift is not clear; elsewhere the cliffs are heavily 
skirted. 
T.ake Awosting lies along a vertical fault j^lane, drift-filled 
at both ends. The fault has not been studied. The north 
wall of the Palniaghat is a vertical fault of 200 feet throw. 
Both these faults seem to be derived from the overthrown 
anticline of the Coxingkill escarpment. ^Iv. Darton is in 
error in declaring the absence of extended faults. 
The next paper was by Dr. A. A. Julien on the "De- 
termination of Brucite as a Rock Constituent." 
After a brief review of the life of Dr. Archibald Bruce, 
of New York city, the discoverer of the mineral, the fact 
of its wide distribution was set forth, both in limestones and 
serpentinoids, either in its unchanged condition or in the 
form of its derivatives, especially magnesite and hydromag- 
nesite, as maintained by Volger in 1855. The following are 
its most marked characteristics for recognition as a rock 
constituent. 
1. In addition to the known basal cleavage, two other 
systems may be distinguished on plates or folia ; that of the 
hexagonal prism, often becoming rhombohedral, intersect- 
ing at 60° or 120° ; and that of the hexagonal pyramid, in- 
tersecting at 90°. 
2. Nemalitic structure or fibration, commonly occurr- 
ing in brucite within serpentinoids subjected to dynamic 
stresses. The major axis of elasticity always lies parallel 
to the direction of the fibers. 
3. Refractive index r.57, sufficient, when the asso- 
ciated minerals are pure, to distinguish it by the Becke 
method from serpentine on the one hand and from amplii- 
boles, dolomite, etc., on the other. 
4. Birefringence [y— ^ = 0.020) presenting interfer- 
ence colors of the upper first order up to skyblue of lower 
second order, in plates or sections of the usual thinness. 
5. Characteristic strain phenomena ; particularly by 
disturbance of the interference figure, examined by con- 
vergent light in basal cleavage plates or folia ; also by a 
variable, small extinction angle in sections parallel to the 
vertical axis. 
