92 CORUNDUM, TTS OCCURRENCE AND DTSTRTBUTTON. 
ORIGIN OF J^MERY IN AMPHIBOLITE AT CHESTER, MASS. 
It is difficult to decide from an examination of the amphibolite 
and the surrounding- rocks at Chester, Mass., whether the amphibolite 
is of sedimentary or igneous origin, for some indications point to one 
and some to the other; hence it is not improbable that some portions 
of this band of amphibolite, which extends across the State, may have 
been formed in one way and other portions in the other way. Profes- 
sor Emerson has examined this belt of rocks for practically its entire 
distance across the State, and Avhile he is inclined to consider the 
amphibolite of sedimentaiy origin, he does not, if I understand him 
correctly, see anything antagonistic to the view that at least a portion 
of it is an intrusive igneous rock. I have examined only that portion: 
of the amphibolite adjacent to the emery vein, and from my observa- 
tions I have come to the conclusion that this part of the amphibolito 
band is an igneous rock that has been intruded into the schists > 
along their line of weakness, and that the magnetite and emery are' 
the first minerals to separate out from this magma. The reasons thatt 
have led me to this conclusion are given briefly in the followin^:^ 
paragraphs. 
The position of the amphibolite, Avith a dip of practically 90° 
lying between the EoAve and the Savoy schists, which would have beer 
a line of least resistance, suggests an igneous origin. AVhat seems tc 
me opposed to the acceptance of a sedimentary origin for this broac 
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 amphibo- 
lite, varying in Avidth from a few feet to 18 feet, Avhich, as far as car 
be judged superficially, is identical in every way with the amphibo 
lite on the west of the vein. Yet, if the amphibolite and emery are o: 
igneous origin, it would naturally be expected that emery would occui 
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 b 
accounted for as secondary minerals. 
Moreover, chromite has l)een found in connection with the serpen 
tine that is associated with the amphibolite, some of which serpentini 
is undoubtedly an alteration of this rock. The presence of the chro 
mite in the serpentine is to me a A^ery good indication of its igneou 
origin, and of its being the alteration product of a basic magnesiai 
rock. This, then, Avould indicate the existence of former masses of ji 
basic magnesian rock Avhicli have been changed into serpentine, th 
serpentinization continuing into the amphibolite for some distance. 
The emeiy is not continuous along the strike of the vein, but occur 
in a series of pockets in the vein that dip about 30° to the north. Ii 
