808 THE GEOLOGY OF MINNESOTA. 



[Zirkelyte. Graywacke. Greenstone. 



Mic. The rounded quartzes are not supplied with an interlocking border, but 

 they frequently show a shadowy extinction due to dynamic fracturing, 

 as well as lines of inclusions. Microcline, as shown by the character- 

 istic twinning, is not uncommon. Other grains are of orthoclase. A 

 single grain of zircon, cut perpendicular to its vertical axis, shows two 

 sets of cleavages, as shown in the figure. In the spaces between the 

 quartzes is a finer debris, largely of quartz, but also containing hornblende and mica. 

 There are also quite a number of pebbles consisting of quartzyte, of which the 

 constituent grains are interlocking in a manner similar to much in the bottom of the 

 Animikie. These grains vary in size, in some of the pebbles being very fine, as in 

 the taconyte, and in others as coarse as in the Pewabic quartzyte. 



The slide represents the finer portion of the conglomerate. One section. 

 Age. Bottom of the Potsdam. N. H. w. 



No. 1905. ZIRKELYTE. 



From the top of the same hill as No. 1902, lying about 200 feet above the gritstone No. 1902. 

 Ref. Annual Report, xxii, page 13. 



Meg. A nondescript rock of green color. Compare No. 2066. 



Mic. In a groundmass of microlitic devitrified glass are numerous idiomorphic 

 fine feldspars and amygdaloidal spaces. The groundmass is mostly chloritized and 

 the amygdaloidal spaces are occupied principally by quartz, some of which shows the 

 optic elongation of quartzine and some of chalcedony, and by calcite. One section. 



Age. Manitou. N. H. w. 



No. 1906. GRAYWACKE. (Grit.) 



"A fine-grained member of the gritstone, from an exposure further east [than Nos. 1902 to 1905]." 

 Ref. Annual Report, xxii, page 135. 



Meg. Fine-grained, compact, hard graywacke. No section. 

 Age. Potsdam. u. s G 



[NOTE. Nos. 1907 to 1941, inclusive, were collected outside of Minnesota. Of 

 these only those of which there are thin sections are here described. Notes on these 

 rocks, Nos. 1907 to 1941, can be found in the Twenty-second Annual Report, pages 

 13-15.] 



No. 1915. GREENSTONE. 



Sample of the more massive portions of the greenstone, Lower Quinnesec falls, Menominee river, Michigan . 

 Ref. Annual Report, xxii, page 13. 



Meg. Apparently a massive rock. 



Mic. Hornblende, composing the larger part, in overlapping, angular, confused 

 fragments, is abundantly mingled with epidote, magnetite (in larger grains), partially 

 altered to leucoxene, some chlorite, some quartz. These lie in a groundmass of fine 

 interlocking secondary feldspar, in which quartz also takes some part. 



