DIORITB INTEUSIVES. 229 



determined as orthoclase by the difference shown by its refractive index 

 and that of the twinned plag-ioclase. Quartz was also recognized in this 

 way in the groundmass. 'The quartz and orthoclase form the cement for 

 the other constitueiits. The muscovite in the groundmass is presumably 

 secondary, as is that in the phenocrysts. (Figs. A and B, PL XL.) 



The rock is here inserted as showing an exceedingly fine grained por- 

 phyritic form of the quartz-mica-diorite. It may compare to this mica- 

 dioi'ite as does the tonalite-porphyrite of Becke^ to the tonalite described 

 by him, and one might call it a quartz-mica-diorite-porphyry. 



No analysis of this rock has thus far been obtainable. Possibly its 

 chemical composition may indicate it to be more closely allied to the true 

 granites than is believed to be the case, judging from its mineralogical 

 composition and its association with the rocks on the border line between 

 granites and diorites. 



SOUTHEAST OF CRYSTAL FALLS. 



Southeast of Crystal Falls, in sec. 16, northwest of Lake Tobin, and 

 extending southwest into sec. 28, T. 42 N., R. 32 W., is a range of hills 

 upon which are numerous exposures of a uniforml}' medium-grained rock. 

 The main mass of the knobs is of tonalite, which shows several facies. A 

 miarolitic texture is very common in this massif. The cavities are now 

 filled with calcite, quartz, and epidote-zoisite alone or together. This last 

 mineral occurs in single large individuals or in tufts of individuals, which 

 radiate from one side of the cavity. In one case a. cavity incompletely 

 filled by such a tuft has been completely filled by a later infiltration of 

 quartz. The color of the rock varies from light pink to very dark greenish 

 gray. The areas of the lighter-colored rocks may be seen extending in 

 finger-like projections into the darker-colored phases. There are, however, 

 no sharp lines between these varieties, but a gradual passage from the lighter 

 to the darker rock. These diff"erent phases evidently belong to a single 

 rock mass. Under the microscope, however, important variations in the 

 textural and mineralogical character of the rock masses are seen. The 

 main mass of the rock is granular tonalite. The essential constituents are 

 plagioclase, orthoclase, quartz, hornblende, and mica. The most common 

 association of minerals is hornblende and mica in automorphic crystals, 



' Petrographisclie studien am Tonalit der Rieserferner, by F. Becke: Tschermaks mineral. 

 Mittheil., Vol. XIII, 1892, p. 43.5. 



