358 
[Assembly 
kind of gneiss contains but little mica, and shows little tendency to de- 
composition, disintegration or alteration, and is therefore well fitted to 
be a good building stone, as regards durability. The same remarks 
also apply to granite, its associate. In the third district, these two 
rocks are of contemporaneous origin. 
The only common metallic mineral which is common in Lewis coun- 
ty to the two rocks, is magnetic or rock iron ore ; this in some places 
is so much intermixed with the rock as to appear like a constituent ; 
but I did not meet with any locality where it was in sufficient abun- 
dance to be worked ; yet it is a fair inference, that where it is found 
disseminated in a rock, it is probable that it may exist in larger parti- 
cles, or workable masses. This ore was noticed near Lewisburgh fur- 
nace, at the High falls also of Black river, at Lyonsdale, and on the 
road from Harris ville to the natural bridge. 
The rock ore may readily be known by its being highly attractive of 
the magnet, by being black in the mass, and black in its powder. It 
forms the iron sand so often met with in the streams and sands of the 
primary region, having been set free by the destruction of its parent 
rock, and subsequently accumulated by the action of water. 
Magnetic iron ore forms the colouring matter, and contaminates, like- 
wise, some of the minerals of the primary rock. The alteration which 
it has undergone in some places, is highly instructive, and corrobora- 
tive of facts, brought forth in the second report, of an action of thermal 
waters at the junction of the primary mass, and its unconformable over- 
lying masses. 
At Little-Falls, the primary rock presents numerous cracks or fissures. 
In some of them the rock, which is usually some shade of gray, is 
observed to be of a bright red for many inches from their surface, and 
sometimes the sides of the fissures are coated by the same red mineral, 
apparently an exudation. This colour is caused by the iron of the 
rock, altered by the action of hot or thermal water, for when the alte- 
ration of the black oxide of iron is unaccompanied by heat, the tenden- 
cy is to the forming of an hydrate, which produces either a yellow or 
brown colour, according to cohesion. That this action has operated to 
a great extent in some other parts of New-York, is evident from inform- 
ation communicated by Prof. Emmons ; for masses w^hich evidently, 
from the structure of their particles, were once magnetic ore, have lost 
