PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 859 



Muscovadyte. Granite.] 



intergrowth is also parallel with the prismatic cleavage (110) and with the ortho- 

 pinacoid 100. The labradorite is fresh, and twinned on the albite and pericline plans. 

 Magnetite is not abundant. One section. 



Age. Cabotian (modified Keewatin). 



Remark. This rock, so evidently a phase of Nos. 2198 and 2199, and of the same 

 rock mass, here plays the role of an igneous rock intrusive on less modified parts of 

 itself. N. H. w. 



No. 2203. MUSCOVADYTE. 



Same place as No. 2202, Disappointment lake. At the gabbro contact. 

 Ref. Annual Report, xxiv, page 62. 



Meg. Inclusion of the greenstone conglomerate in the gabbro. 



Mic. The structure is granulitic, the fe Idspars averaging larger than any of 

 the other minerals, which they embrace poikilitically. They show the albite and 

 pericline twinning and are fresh and clear. Rarely, however, a hypersthene surrounds 

 a few globular feldspars. The hypersthene is fresh and is distinguished from ensta- 

 tite by having n v for acute bisectrix perpendicular to 100. Besides the foregoing the 

 rock only embraces a very few grains of magnetite and one or two of biotite. One section. 



Age. Archean (Keewatin). 



Remark. It is to be noted that the rocks Nos. 2202 and 2203 mineralogically are 

 very nearly allied, but petrographically and structurally they are contrasted. The 

 The former is a characteristically crystalline gabbro with large plates of hypersthene 

 (interleaved with diallage), showing a partial ophitic structure. The latter is granu- 

 litic, the hypersthenes being small and round, and not infrequently wholly surrounded 

 by the feldspar. The species of the feldspar can be safely taken to be labradorite 

 in both cases, but no reliable measurement of extinction is obtainable in' No. 2203. 

 Were it not that the rock No. 2203 can be followed continuously from the gabbro 

 border northward to the iron ore pits (but few rods, No. 2201) and thence to the lake 

 shore at Cheadle's cabin (Nos. 2197 and 2198), and that throughout it is uniformly 

 the same rock, often pebbly, distinctly stratified, and is thence demonstrably the 

 same formation as the pebbly greenstone, often called conglomerate, it would be haz- 

 ardous to affirm, in the face of numerous published statements to the contrary, that 

 these are the same rock genetically, structurally and historically. The similarity of 

 the metamorphosed clastic to the contacting gabbro is to be explained by the great 

 a priori similarity of the greenstone debris in chemical composition to a basic 

 igneous rock, and leads to the reasonable assumption that the gabbro itself is the 

 end product of such metamorphism. N. H. w. 



No. 2207. GRANITE. (EstereUyte. ) 



Central part of see. 32, T. 64-8, Disappointment lake. 

 Ref. Annual Report, xxiv, page 64. 



Meg. Dike of red granite cutting the gray pebbly rock. 



