THE MINERALOGRAPHY OF THE FELDSPARS Dolel 
which are composed largely of hornblende. These are hornblende 
schists, or paramphibolites. In a similar manner old basic erup- 
tives, such as diabases, diorites, and gabbros, are metamorphosed to 
orthoschists and gneisses. They may be called ortho-amphibolites. 
“The origin of the amphibolites is a question of the highest impor- 
tance in the elucidation of the geology of the [Haliburton and 
Bancroft] area, as well as one of great interest from a petrographical 
standpoint . . . . [examination has] shown, beyond a doubt, that 
amphibolites, which, in many cases cannot be distinguished apart, 
have been produced by the action of granitic instrusions on lime- 
stone. There is also reason to believe that other amphibolites 
have been produced in still other ways [for there is evidence that 
some amphibolites are] of undoubted igneous origin.’ 
In the Adirondack Mountains ortho- and paramphibolites 
present a difficult problem. In areal mapping this problem is of 
scientific interest only, but when these are encountered upon 
mining properties the distinction between the two types becomes 
essential. The writer has encountered amphibolites? where it was 
impossible to classify the rock. In some doubtful cases the rocks 
were studied petrographically. 
Petrographic Study and Inter pretation.—Specimens were collected 
from rock masses where field relations pointed to a definite origin. 
Microscopic examinations revealed striking similarities and a few 
differences. ‘The similarities need not be touched upon; it is the 
latter that are important. If the rock is sedimentary in origin 
and derived from calcareous shales as Cushing suggests,} quartz 
would be expected to occur, as unmetamorphosed shales almost 
universally carry some quartz. Thus if any original quartz is 
present in an amphibolite, it gives it a sedimentary look, for basic 
(subsalic, femic) rocks are usally lacking in this mineral. On 
the other hand the absence of quartz suggests an igneous origin, 
tF..D. Adams and A. E. Barlow, Canada, Dept. Mines, Geol. Surv. Mem. 6, 
pp. 158-59, IgIo. 
2See J. F. Kemp and H. L. Alling, ‘The Geology of the Ausable Quadrangle,” 
New York State Mus. Bull. (In preparation.) 
3H. P. Cushing, New York State Mus. Bull. 169, p. 19, and Bull. ror, p. 15, 
IQI4. 
