PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 113 



Porphyryte. Quartz.) 



Meg. The minerals in the specimen are promiscuously arranged, and seem to 

 have been deposited secondarily in a porous rock, much of which has been changed 

 into these minerals. The rock is spotted coarsely with white (calcite and quartz), 

 green and yellow (chlorite, epidote) and red (which is perhaps orthoclastic). There 

 are also considerable areas of a gray or dun-colored aphanitic rock, not so durable as 

 the red mineral. 



Mic. The foregoing named minerals all appear in the slide. The red mineral 

 appears in the prepared slide as an irregularly disseminated reddish powder, scattered 

 sometimes abundantly, but without crystalline form. Some of its crystalline grains, 

 taken from other parts of the specimen, subjected to hydrofluosilicic acid, give 

 crystallites of potassium fluosilicate, large and abundant, with some of lime, but 

 none evident of soda, thus proving the orthoclastic character of the feldspar. 



In another part of the slide a portion of the original rock is seen. It is finely 

 sprinkled with slender microlites of & feldspar that is lath-shaped, but tapering at 

 the extremities. In immediate connection with this are also a number of large 

 apatites, showing their characteristic transverse fissuring. The great thickness of 

 the section is shown by the fact that these apatites gave colors of double refraction, 

 viz.: yellow and red. The slide must therefore have a thickness of about .08 

 millimeters, according to the scale of Newton's colors, given by Levy and Lacroix.* 



One section examined. N. H. w. 



No. 12. PORPHYRYTE. ( Amyydaloidal. ) 



Duluth. Extending forty-nine paces along the lake shore, east of No. 11. 

 Ref, Annual Report, ix, page 13. 



Mey. A brown amygdaloid, containing numerous porphyri tic plagioclases. The 

 amygdules are filled with epidote, chlorite and quartz. 



Mic. Essentially the same as Nos. 7BC, 9 and 11, except in two particulars. 

 First, the feldspars are in part replaced by chlorite, and second, the lath-shaped 

 plagioclases of the groundmass are fewer in number. 



One section examined. 



Age. Cabotian. u. s. G. 



No. ISA. QUARTZ. (Nodule.) 



Duluth. A large nodule from No. 12. 

 Ref. Annual Report, ix, page 13. 



Meg. The quartz is white and grayish, having a fibrous structure that radiates 

 from a centre, but a coarse banding somewhat agate-like, but without coloration. Its 

 radiated and banded structure shows the continued growth of crystals from a common 

 point, with some variations in the environment. The central nucleus of the mass, 



* Miniraux des Roches (plate). 

 9 



