THE UPPER SLATE MEMBER. 339 



evidenced by rounded cores and secondary enlargements. The biotite of 

 tlie matrix is in part plainly secondary to feldspar and is precisely like that 

 found in the larger feldspars The folia are deep Ijrown and vcrv strongly 

 dichroic, and therefore probably bear a large percentage of iron. Doubt- 

 less much of this matrix is due to the decomposition of fragmental feldspars, 

 which were smaller than tliose which yet remain partly mialtered, and 

 which have therefore completely altered to mica and (piartz. 



Tlie feldspar plainly shows this rock to liave been fragmental, and the 

 alteraticm of felds})ar to both biotite and muscovite upon a large scale is 

 most beautifully shown. The large quantity of dark l)ro\vn and l)1ack 

 ferrite has doubtless furnished the iron required for the formation of the 

 l)iotite. The peculiar spotted appearance of the sections, the distinctness of 

 the spots varying with the freshness of the feldspar, viewed without the 

 microscope, gives a clear idea, when taken in connection with their appear- 

 ance under the microscope, of the processes Ijy which the rock reached 

 its present c«^udition. 



This "rock must as deposited have been a toleral)ly coarse grained one, 

 many of the larger, fresher feldspars averaging about 1 mm. in diameter, 

 but the alteration of each of these most changed feldspars in the specimens 

 to a vast number of mica folia and (piartz grains has caused the rock to 

 become exceedingly fine grained, there remaining, however^ perfect proof 

 even in these specimens of the original rounded fragmental character of 

 some of the feldspar. 



(5) Nearlfi crijstaJUne muscovitic hiotite-schist (PI. xxxiv. Fig. 1). — Macro- 

 scopically, this rock is Hue grained, grayish, and quartzose, with small mica 

 flakes visible. Under the microscope the thin section shows a tine grained 

 groundmass of quartz and feldspar in which are abnmlant nuiseovite and 

 biotite. ^lany of the (juartz grains include numerous folia of mica. The 

 feldspar areas include (piartz and b(ith nuiscovite and biotite. This section 

 Ijy itself if not examined closely would be taken to be of a completely 

 crystalHne mica-sciiist in which the interlocking and mutual inclusions of 

 the different minerals are of the most intricate kind, but, like the other 

 mica-.schists of the I'enokee series, it is an ordinarv clastic rock in which 

 the inetasomatic changes have gone very far. The large areas of quartz, 



