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have added to the apparent thickness. One feels rather quizzical at 
the confidence with which they present this figure. 
The least metamorphosed, pure limestones are mostly fine-grained, 
and of a bluish color, the dark color being due to organic matter. They 
are interstratified with dolomitic and magnesian beds, but the pure lime- 
stones dominate. This great development of pure limestones modifies 
the prevailing opinion that the pre-Cambrian does not contain many 
pure limestones. 
The magnesium carbonate is found to occur in the mineral dolomite, 
and as a slight replacement of calcium carbonate in calcite. All grada- 
tions are also found, from pure limestones to argillaceous and quartz- 
bearing limestones and dolomites, quartzites and paragneisses, but the 
impure rocks are highly recrystallized as a rule. Most of the limestones 
and dolomites have been intensely metamorphosed, but the relative 
importance of intrusion and of intense folding in bringing this about is 
not always clear. The carbonates have adjusted themselves to stress 
strain conditions by plastic flow, including recrystallization, gliding, 
shearing, and granulation. In general the limestones were more plastic 
than the silicate or quartz rocks associated with them, flowing in between 
the fragments of the latter. 
The most apparent effects of metamorphism on the limestones are 
a coarsening of the grain, and a change of the color from blue to white, 
and the development of graphite, complex silicates, and heavy oxides. 
Pyroxenes, amphiboles, feldspar, epidote, quartz, scapolite, magnetite, 
spinel, and many others are more or less common developments. There 
is also a notable series of sulphides, among them mispickle, galena, 
pyrite, molybdenite, orpiment, and realgar. To what extent these new 
developments represent recrystallization of materials originally present 
and to what extent infusion of new materials is not clearly determined. 
Where the limestones have been invaded by basic intrusions and, on a 
much larger scale, where they have been invaded by the granite gneisses, 
they have been altered either to a granular aggregate of pyroxenes and 
hornblende, with scapolite and accessory minerals, or to a feather-like 
aggregation of hornblende with other silicates and residual calcite and 
dolomite. To these rocks, approximating the composition of diabase, 
_the name of amphibolites have been given. The granite gneisses are 
also full of amphibolite inclusions most of which probably represent 
altered limestones. From a comparison of the composition of three 
amphibolites, representing different stages of the alteration of limestones 
by granite gneiss intrusion, the authors conclude that the process of 
