276 HAROLD L. ALLING 
The writer has already discussed this phenomenon and has reached 
the conclusion that pressure does not produce microcline from 
orthoclase; it only starts and accelerates the change. This sug- 
gestion seemed a promising method of attack. Unless the graphite- 
bearing schist had suffered very severe regional metamorphism the 
potassic feldspar would still exist in the metastable condition which 
we know as orthoclase. But under the stress and jar of faulting 
the feldspar would take on the microclinic type of twinning as a 
consequence of the inversion of orthoclase to microcline. Micro-- 
scopic examination would locate the fault. | 
Another trip into the field resulted in a collection of a suite of 
specimens from numerous localities in and about the old workings. 
Petrographic Study.—Examination of the slides from these 
specimens showed that the writer’s supposition was entirely 
correct. Some were composed of orthoclasic feldspar while others 
showed microclinic types. 
Inter pretation.—It was concluded from quantitative microscopic 
analyses that specimens which showed a high orthoclasic content 
came from areas that were free from faulting, and that specimens 
showing soda microcline were situated in zones affected by faulting. 
Resulis.—In 1918, the following year, the writer took his map 
of the Rock Pond workings, giving the results to the petrographic 
study, back into the field and erected piles of stones where faulting 
had been deduced from the slides. From the position of these 
cairns it was possible to trace a group of faults that cut off the ore 
on three slides. Careful examination of the walls of the pits 
revealed conclusive evidence of the correctness of the interpretation. 
Slickensides and breccias were where the microscope had indicated 
that they should be. 
CASE TWO—ORTHO-AMPHIBOLITES VERSUS PARAMPHIBOLITES 
The Problem.—In many pre-Cambrian areas where ancient sedi- 
ments have been invaded by igneous rocks, and subjected to con- 
tact and regional metamorphism, the character of the original 
rocks becomes profoundly altered, both in regard to mineralogical 
and structural relationships, under these forces. Both limestones 
and calcareous shales become metamorphosed into paraschists, 
