PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 373 



Slate. Graywacke.J 



which was produced probably at the same time and by the same pressure that gave 

 the argillyte its slaty cleavage. 



Mic. The quartzes are sub-rounded and angular, and have a conspicuous 

 undulatory extinction, indicative of incipient crushing. The feldspars are usually 

 crowded with microliths and clouds of impurities which render them unidentifiable. 

 These are probably of orthoclase, since others, which are plainly striated, and which 

 must have undergone the same general history, are well preserved, though not of 

 secondary origin. The fine matrix is composed of a gray or greenish-gray chloritic, 

 ill-defined substance which probably represents simply the finer debris from the same 

 source as the coarser grains. There is in it more or less of opaque dust. 



One section. 



Age. Animikie(?) N. H. w. 



No. 457. SLATE. ( Grayivackenitic. ) 



"Slate; average sample for the rock of the lower falls." Same locality as No. 453. 

 Ref. Annual Report, x, pages 12, 21. 



Meg. A black, rough, slaty rock. The mass of the rock is dark, apparently 

 siliceous and very fine grained. In it can be seen small scattered grains of quartz 

 and some of feldspar. 



Mic. The section shows grains of quartz and feldspar embedded in a dirty 

 greenish groundmass. These grains vary from those which are rounded to those 

 which are sharply angular. The feldspar is usually somewhat kaolinized. Many of 

 the grains show no twinning, while a few show fine twin lamellae. As a rule cleavage 

 is not noticed, and the species of none of the feldspar grains was determined. The 

 groundmass, under a high power, is seen to be made up of minute flakes of chlorite 

 and sericite, among which are fine grains of probably both quartz and feldspar. There 

 are also many opaque black to grayish specks throughout the groundmass. One 

 small fragment thought to be rutile was seen, also apparently a fragment of brown 

 hornblende. A few small epidote grains were seen also. 



One section. 



Age. Animikie(?) u. s. G. 



No. 458. GRAYWACKE. 



From the clastic beds adjoining a dike at the lower falls of the St. Louis river, S. E. # sec. 10, T. 48-16 W. 

 The clastic rocks here have a coarsely basaltic structure along the east side of the dike, which is at least twenty 

 feet wide (No. 459). The columns run east-southeast and dip about 30 from the horizon, while the dike runs 

 north-northeast. 



Ref. Annual Report, x, pages 12, 13; Annual Report, xxiii, page 239. 



Mac. There are two hand specimens. One is a gray, hard, compact rock with 

 an almost aphanitic background in which are numerous small quartz grains and gray 

 and pink feldspar grains. This specimen is perhaps a fair representative of what 



