316 Exploring Visits to the Sources of the Hudson. 
Hudson near the head of tide-water, for the fragmentary remains of 
the labradoritic rock, but hitherto without success. We may hence 
infer that the whole amount of this rocky material, which, aided by 
the ice, and the powerful impulse of the annual freshets, finds its 
way down the Hudson, a descent of from two thousand to four thou- 
sand seven hundred feet, is reduced by the combined effects of air, 
water, frost, and attrition, to an impalpable state, and becomes im- 
perceptibly deposited in the alluvium of the river, or continuing sus- 
pended, is transferred to the waters of the Atlantic. 
Great Trap Dyke. 
On the 7th of August we visited Avalanche Lake, and exam- 
ined the great dyke of sienitic trap in Mount McMartin, which cuts 
through the entire mountain in the direction from west-northwest to 
east-southeast. This dyke is about eighty feet in width, and being 
in part broken from its bed by the action of water and ice, an open 
chasm is thus formed in the abrupt and almost perpendicular face of 
the mountain. ‘The scene on entering this chasm is one of sublime 
grandeur, and its nearly vertical walls of rock, at some points actu- 
ally overhang the intruder, and seem to threaten him with instant 
destruction. With care and exertion this dyke may be ascended, by 
means of the irregularities of surface which the trap rock presents, 
and Prof. Emmons by this means accomplished some twelve or fif- 
teen hundred feet of the elevation. His exertions were rewarded - 
by some fine specimens of hypersthene and of the opalescent lab- 
radorite, which were here obtained. ‘The summit of Mount McMar- 
_tin is somewhat lower than those of the two adjacent peaks, and is 
estimated at four thousand nine hundred and fifty feet above tide. 
The distance from the outlet of Lake Colden to the opposite ex- 
tremity of Avalanche Lake is estimated at two and a quarter miles. 
The stream which enters the latter at its northern extremity, from 
the appearance of its valley, is supposed to be three-fourths of a 
mile in length, and the fall of the outlet in its descent to Lake Col- 
den is estimated, as we have seen, at eighty feet. ‘The head waters 
of this fork of the Hudson are hence situated farther north than the 
more remote source of the Main Branch, which we explored on the 
Ath and 5th, or perhaps than any other of the numerous tributaries 
of the Hudson. ‘The elevation of Avalanche Lake is between two 
thousand nine hundred and three thousand feet above tide, be- 
ing undoubtedly the highest lake in the United States, east of the 
Rocky Mountains. 
