PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 349 



Clay slate. Porphyry.] 



No. 392. CLAY SLATE. 



Same locality as No. 389. 



Ref. Annual Report, ix, page 97. (See remarks under No. 389.) 



Meg. A very fine-grained, dark gray, clay slate. 



Mic. The section is similar to No. 390, but is much finer grained and contains 

 little or no calcite. It is composed essentially of very minute flakes of chlorite and 

 sericite, and minute grains, probably of quartz, with also probably feldspar. 



One section. 



Age. Archean (Keewatin). u. s. G. 



No. 393. PORPHYRY (?) (Sheared.) 



Same locality as No. 389. 



Ref, Annual Report, ix, page 97. (See remarks under No. 389.) 



Meg. A gray rock, consisting of small (about one twenty-fifth of an inch 

 across), white, crowded feldspar crystals in a sparse, very fine-grained, darker 

 groundmass. 



Mic. In section the crowded feldspar crystals are not as distinct as in the hand 

 specimen, for they are much altered and clouded and are composed largely of almost 

 the same materials as found in the groundmass. The species of the feldspar cannot 

 be determined. Nearly all the grains show no twinning, but a few show traces of 

 twinning striae, and one is a simple twin, apparently according to. the Carlsbad law. 

 In shape, these feldspars are rounded or subangular, and others are partly idiomorphic. 

 In reflected light they appear gray and semi-opaque, and this character is also found 

 irregularly in the groundmass. The groundmass is composed of flakes of sericite and 

 chlorite, and small grains of quartz with probably feldspar also. The sericite is 

 developed especially often in cracks of the feldspars and between the different grains, 

 and the flakes sometimes have their long axes perpendicular to the surfaces of the 

 feldspar grains. Crossing the section is a small vein-like form composed mainly of 

 small grains of quartz, and similar quartz occurs elsewhere in the slide. 



One small section. 



Age. Archean (Keewatin). 



Remark*. The exact nature of this rock is not evident. The feldspars may 

 have been crystals in an acid eruptive, now much altered, or it is quite possible that 

 they are clastic in their origin, being either true waterworn grains or else of a 

 tuffaceous origin. That these feldspars have developed in situ, seems rather improb- 

 able when we consider their highly altered characters; at least, if they did develop 

 in situ, they have been much decayed since. u. s. G. 



