pratt.] MODES OF OCCURRENCE. 25 
netite to others where there is an intimate admixture of magnetite 
and corundum. At the Sackett mine (described on page 68) the 
corundum, of a bronze color and luster, is coarsely crystallized, giv- 
ing the ore a porphyritic appearance. Sometimes the corundum has 
crystallized out in blue and white crystals and in masses of a pound 
or two in weight. 
ORIGIN OF THE EMERY. 
It is a perplexing question to decide from the examination of this 
and the surrounding rocks whether the amphibolite is of sedimentary 
or igneous origin, for there are many indications that point to one and 
to the other. It is not improbable that some portions of this band 
of amphibolite, which extends across the Stale, may have been formed 
in one way and other portions in the other. Professor Emerson has 
examined this hell of rocks for practically its entire distance across 
the Slate, and while he is inclined to consider this band of amphibolite 
of sedimentary origin, lie does not, if 1 understand him correctly, see 
anj 7 thing antagonistic to the view thai at Least a portion of this amphi- 
bolite is an intrusive igneous rock. I have examined only that por- 
tion of the amphibolite adjacenl to the emery vein, and from my 
observations I had come to the conclusion thai this pari of the 
amphibolite hand was an igneous rock that had been intruded into 
the sehisls along their line of weakness, and thai the magnetite and 
emery had been llie lirsi minerals to separate out from this magma. 
The reasons thai have led me to this conclusion are given briefly 
below. 
The position of the amphibolite, with a dip of practically 90°, lying 
between the Rowe and Savoy schists, which would have been a line 
of leasl resistance, suggests an igneous origin. What seems to me 
opposed to the acceptance of a sedimentary origin for this broad 
band of amphibolite is the separation of the vein of emery and mag- 
netite from the sericitic schist on the east by a band of the amphibolite 
varying in width from a few feet to IS feet, which, as far as can be 
judged superficial ly, is identical in every way with the amphibolite on 
the west of t lie vein. Yet, if the amphibolite and emery are of igneous 
origin, it would naturally be expected that emery would occur on the 
west side of the amphibolite, but none has been found there, although 
it is not certain that it does not occur. 
Only a small amount of carbonates has been found in connection 
with the amphibolite and emery — no more than could readily be 
accounted for as secondaiy minerals. 
Then, again, chromite has been found in connection with the ser- 
pentine that is associated with the amphibolite, some of which is 
undoubtedly an alteration of this rock. The presence of the chromite 
in the serpentine is to me a very good indication of its igneous origin, 
aud its being the alteration product of a basic magnesian rock. This, 
