PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 95 



Granite. Diabase.] 



meters long, still they are much larger than the other constituents, and are almost 

 completely idiomorphic. Most of these crystals show no polysynthetic twinning, 

 although in two of them traces of it are visible. An attempt to get cleavage flakes 

 for the measurement of the extinction angle was unsuccessful, as the crystals are 

 quite small and usually considerably altered to a pale-greenish chloritic substance. 

 The porphyritic feldspars are rarely reddened as is the feldspar of the groundmass. 

 No porphyritic hornblende is shown in the slide. 



The groundmass is composed of feldspar, hornblende, quartz and magnetite. 

 The feldspar is much altered and reddened, as is so common in the acid granular 

 rocks of the Cabotiau, and shows no twinning striations, and the cleavage is ill devel- 

 oped; it is very probably orthoclase. Quartz occurs in sharply defined grains and 

 also in micropegmatitic mtergrowths with the feldspar. This micropegmatyte per- 

 vades almost all the feldspars, and at times becomes almost granophyric. As to how 

 much of this structure is secondary it is impossible to say. The groundmass is, how- 

 ever, holocrystalline and is finely granitic in structure. Hornblende, of the usual 

 green variety, is quite common; it occurs in allotriomorphic grains and in minute 

 flakes, which have a slightly bluish tinge and are scattered throughout the rock. 

 Some of the areas of hornblende appear partially granular and much resemble horn- 

 blendes that are secondary after pyroxene, but in no case was any pyroxenic material 

 seen nor suggested by the outlines of the hornblende areas. Magnetite is quite 

 abundant; it occurs in a few irregular masses two to three millimeters across and in 

 numerous minute idiomorphic grains. 



One slide examined. 



Age. Cabotian. 



lie ma rl-fi. The rock is a brownish red, very fine grained, sub-porphyritic, horn- 

 blende granite. "This [No. 3] spreads wider and is to be seen at other points back 

 of Duluth, yet appears rather to be in patches, or in veins in other rock. At New- 

 son's quarry a similar red rock penetrates the gray rock in seams, and occupies a 

 larger area in the lower part of the quarry." * u. s. G. 



No. 4. DIABASE. (Coarse, decayed.) 



Duluth. Near the Union station. 



Kef. Annual Report, ix, page 11; Annual Report, x, page 41. 



This is a much decayed, light gray to flesh-colored r'ock of medium grain. 

 Feldspar makes up more than half the rock; it is whitish to pinkish in color and 

 appears like decayed orthoclase. Several larger crystals occur in the hand specimen; 

 these have a greenish shade and are one-half to three-fourths of an inch across, but 

 they are not sharply marked off from the rest of the rock and so do not give it a 



*N. H. WlNCHELL. XlnthAilliinil ]!< jiiii-t, p. 11. 



