ORIGIN OF AMPHIBOLITES OF LAURENTIAN AREA 9 
fragments sometimes have somewhat flowing outlines as if they had 
been subjected to a certain amount of movement when in a softened 
condition. 
When examined in thin sections under the microscope the limestone 
which is in the act of passing into amphibolite is seen to do so by the 
development in it of certain silicates. These, when the change is 
complete, are so abundant that they have entirely replaced the 
calcite while in the intervening stages some of the original calcite still 
remains. These silicates belong to the following species: Pyroxene, 
hornblende, sphene, scapolite, plagioclase, microcline, orthoclase, 
and quartz. The relative abundance of these minerals varies in 
different bands and from place to place in the rock. ‘Their characters 
are as follows. 
The Pyroxene is rather deep green in color and shows an absence 
of pleochroism. It is one of the chief constituents, being in the earlier 
stages of the change present in large amount. It first appears in 
individuals which are rounded in shape, do not possess crystallographic 
outlines or any approximation to crystalline form. In those varieties 
rich in calcite, the sections of the pyroxene grains are frequently nearly 
circular. 
The Hornblende, which at first is much less abundant than the 
pyroxene, is also green in color but it is a much deeper green than 
the pyroxene. The grains are similar to those of the pyroxene in 
form, but are usually less rounded. It is intimately associated with 
the pyroxene, often forming adjacent grains, but there is no conclusive 
evidence that one mineral is derived from the other. It is strongly 
pleochroic. 
The Sphene is present only in a very small amount in the form of 
small rounded grains of a brown color. 
Scapolite is usually present in considerable amount. It polarizes 
in brilliant colors, is uniaxial and negative, and shows the other 
microscopical characters of this mineral. 
The Feldspars vary greatly in amount. In places they form a 
considerable part of the rock, while no scapolite is present. In 
other places the scapolite seems to take their place and they are 
-reduced to the rank of accessory constituents. All three varieties 
of feldspar mentioned often occur in the same specimen, their relative 
