838 THE GEOLOGY OP MINNESOTA. 



[Granite. Clay slate. 



out a rude schistose structure parallel with that in the slates. It is impossible to 

 state whether these alternations of rock indicate a coming on of igneous rock, inter- 

 stratified with the Coutchiching, or that the strata all belong to the Coutchiching, 

 but all the appearances, both in the field and under the microscope, point to the 

 gradual transition from the non-crystalline to the crystalline. 



A similar alteration is seen again at the western end of the lake (Nos. 371 and 

 372), near the river, where the route leaves the lake westward. The bedding is here 

 quite marked, running more south, and dipping at a high angle toward the northwest. 



Besides this gradual transition, there is to be noted a sudden transition, which 

 occurs when eruptive granite of a date later than the schists has invaded the 

 schists, in the manner described by A. C. Lawson. Such rocks are strictly not 

 of the same age, although they have commonly been together called Laurentian. 

 (Compare Part I, page 27, et seq.) N. H. w. 



No. 372. GRANITE. ( Chlaritic. ) 



Same locality as No. 371. 



Ref. Annual Report, ix, page 95. 



Meg. A rather fine-grained, pink granite composed of quartz, a pinkish feldspar 

 and a little chlorite. 



Mic. The section shows a granite composed largely of quartz and feldspar, with 

 a little chlorite and epidote. The feldspar is quite extensively kaolinized and much 

 of it shows no twinning lamellae. There are, however, a number of grains with fine 

 twinning lamellae and very low extinction angles. While no distinctive characters 

 were determined, it seems that the feldspar is orthoclase and anorthoclase or oligoclase. 

 One grain, cut perpendicular to the positive bisectrix, gave an extinction of about 7. 

 The quartz and the less altered feldspars show pronounced undulatory extinction, 

 and there is one vein-like band, crossing the rock, which is made up of fine grains 

 of quartz and feldspar evidently formed by a crushing and shearing of the rock 

 along this line. 



One section. 



Age. Archean (near Coutchiching). u. s. G. 



No. 373. CLAY SLATE. 



Northeast end of Mud lake; N. W.J4 sec. 2, T. 62-14 W. 

 Ref. Annual Report, ix, page 95. 



Meg. A dark aphanitic rock, appearing like a hardened clay slate; rather soft; 

 crossed by minute white veins probably composed of quartz and calcite. 

 No section. 

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



