348 THE GEOLOGY OF MINNESOTA. 



[Clay slate. Graywacke. 



Remarks. We are forced to conclude that by some means a secondary feldspar 

 and siderite have been developed to the almost total exclusion of the original feldspar. 

 The dynamic stress through which the rock, in common with all the rocks of the 

 region, has passed, is probably responsible for these new minerals. N. H. w. 



No. 390. CLAY SLATE. 



Same locality as No. 389. 



Ref. Annual Report, ix, page 97. (See remarks under No. 389.) 



Meg. A rather soft, gray or greenish-gray, aphanitic clay slate. 



Mic. The section is very fine grained. It is composed essentially of calcite, 

 minute flakes of sericite (or muscovite) and chlorite, and a very minutely granular, 

 rather indistinct aggregate which probably contains both quartz and feldspar. 



One section. 



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



No. 391. GRAYWACKE. 



Same locality as No. 389. 



Ref. Annual Report, ix, page 97. (See remarks under No. 389.) 



Meg. A light gray, or greenish gray, fine-grained rock. A few grains of quartz 

 and occasionally a feldspar are all the minerals that can be seen macroscopically. 

 The rest is a flaky, indistinct, talc-like (but harder than talc) base. 



Mic. The section shows scattered grains, rounded and subangular (sometimes 

 quite angular), of quartz and feldspar in a much finer grained background. The 

 quartz grains are not sharply outlined, but their peripheries are closely cemented to 

 and interlocked with the small grains of the groundmass. The feldspar is clouded 

 and kaolinized and is not always very distinct from the groundmass. The species of 

 the feldspar could not be determined, although it seems likely to be largely orthoclase. 

 No polysynthetic twinning lamellae were seen, but there were indications of a few 

 simple twins, apparently by the Carlsbad law. The groundmass is composed of a 

 fine-grained aggregate, similar (but a little coarser grained) to No. 390. Caf<-if<-, 

 sericite, chlorite, quartz, and probably also feldspar, form the chief constituents of 

 this groundmass. There are collections of very minute grains, which cannot be 

 determined, but which are perhaps epidote. 



One section. 



Age. Archean (Keewatin). 



Remarks. The similarity of the groundmass of this rock to others from Ver- 

 milion lake, the slates and schists, is quite decided. Without the larger grains of 

 quartz and feldspar the rock could be termed a clay slate or perhaps a sericite schist. 

 It is also to be noted that in many respects this groundmass resembles the ground- 

 mass of the highly sheared and altered " quartz-porphyries." u. s. G. 



