52 ANNALS NEW YORK ACADEMY OF SCIENCES 



Garnet Rocks 



On the southern border of the district, along the road at the foot of 

 Salt Hill, is found a contact facies of very interesting composition. The 

 outcrops are badly cracked and jointed, and the rock is friable and stained 

 brown along these cracks. \Vhen fresh, two varieties may be distin- 

 guished, one pink and the other an ash gray, these two occurring within 

 30 yards of each other. 



The former variety is of about the color of rhodonite ; it is fine grained, 

 but is thickly penetrated by flat gray tremolite rods. Under the micro- 

 scope the pink mineral, which makes up more than half the rock, is seen 

 to be garnet. It is practically colorless and always isotropic and is, prob- 

 ably grossularite. The tremolite 37 is typical and constitutes nearly one 

 third, and quartz makes up the rest. Eutile, in small sharp grains, with 

 very high relief and birefringence, and a deep yellow color, is quite com- 

 mon. This rock is thus probably derived from an impure limestone, or 

 possibly a calcareous schist, and presents an interesting contrast to the 

 wernerite rock. The abundance of titanic oxide present is a curious 

 feature in the contact metamorphism of the limestones ; either as ilmen- 

 ite, titanite or rutile, it appears to be always present, whereas iron and 

 alumina are more typical in the case of the mica schists. 



The gray rock in thin section is resolved chiefly into quartz and garnet. 

 The latter is identical with that in the last, except in its color in mass. 

 The most interesting and suggestive thing about this rock is the presence 

 in rather small amount of both corundum and pleonaste; and magnetite 

 is, moreover, thickly scattered over the slide. Notwithstanding its pro- 

 pinquity to the previous phase, the aluminous character of its components 

 seem to indicate derivation from a mica schist rather than a calcareous 

 rock. 



The very abnormal developments which are associated with the emery . 

 may or may not be contact rocks ; they had best be described, therefore, 

 in conjunction with the emery. 



INCLUSIONS 



Inclusions 38 of schist (and occasionally of limestone and gneiss) have 

 been alluded to frequently in the previous pages ; they are quite abundant 



37 Kemp mentions an analogous rock from the border of the Rosetown extension, com- 

 posed entirely of tremolite (Op. cit., p. 252.). 



38 Accidental xenoliths of this kind are of wide occurrence in other igneous districts. 

 They have been exhaustively studied by Lacroix, "Les Enclaves Des Roches Volcan- 

 iques," 1893. The tendency of intruded rocks to carry them is discussed in "Geology of 

 the Castle Mountain Mining District, Montana," Bull. U. S. G. S., 139. 1896. 



