ORIGIN OF AMPHIBOLITES OF LAURENTIAN AREA 7 
a) The alteration of the limestone into masses of granular greenish 
pyroxene rock, usually containing scapolite, or into a rock consisting of 
a fine-grained aggregate of scales of a dark-brown mica. 
6) Intense alteration of the limestone along the immediate contact 
into a pyroxene gneiss or an amphibolite. 
c) In addition to these alteration products, in certain cases the 
granite dissolves or digests the invaded rock, after having altered 
it in one or other of the ways above mentioned. 
The alteration products of Class a may be considered as due to 
the heated waters or vapors given off by the cooling magma, that 
is to be of pneumatolitic origin, while the alteration products of 
Class b result from the more immediate action of the molten magma 
itself. The products of these two classes of alteration, however, 
have much in common and naturally pass into one another. 
The most common product of the alterations of Class ais a granular 
pale-green pyroxene rock which occurs in the limestone at or near the 
contact with the granite. This pyroxene rock, resulting from the 
alteration of the limestone, varies considerably in texture from place 
to place, but is usually medium in grain and granular in character, 
the sahlite individuals of which it is composed being short and stout 
with a hypidiomorphic development. Associated with the pyroxene 
in this rock are black mica, hornblende, scapolite, epidote, garnet, 
sphene, spinel, zircon, tourmaline, pyrrhotite, pyrite, molybdenite, 
calcite, apatite, and occasionally quartz and feldspar, as accessory 
constitutents. Of these minerals the mica and hornblende especially 
have a tendency to occur in segregations or nests composed of very 
large individuals, so large in fact that the mica has been mined in 
these pyroxene rocks at several places in the area, in one case mica 
crystals, having cleavage surfaces measuring two feet by two feet 
and a half, having been obtained. ‘The calcite, when present in the 
rock, is usually in the form of very coarsely crystalline aggregates, 
cementing the other constituents together and into which the other 
minerals grow in the form of perfect crystals with excellent termina- 
tions. This calcite represents portions of the original limestone 
which have survived in an unaltered condition, except that they have 
grown more coarsely crystalline. When the calcite is subsequently 
removed in solution by percolating waters, spaces result which when 
