PETROGRAPHICAL CHARACTERS OF HEMLOCK FORMATION. 443 



only members of the area which possess undoubted bedding. The plane of 

 contact between an amygdaloid and a layer of greenstone-conglomerate in 

 SE. i sec. 10, T. 44 N., R. 31 W., dips eastward at an angle of 50°. 



Two well-marked systems of cleavage traverse all the rocks of the 

 southern river section. The angle betv/een their strikes is always acute 

 toward the north, varying from 5° to as high as 34° in different exposures, 

 while the direction of the bisectrix is almost constant at N. 8°-10° W. 

 The dip of both systems is toward the east at about the same angle, 

 namely 60° to 60°. The two systems are usually both well developed, so 

 that the outcrop edges break down by weathering along zigzag lines. 

 The character of the cleavages varies from fine partings which divide the 

 surface into rhombs, sometimes extremely regular in the more aphanitic 

 rocks to a single perfect scliistosity capable of minute subdivision, along 

 which the component minerals are visibly aligned, in the more crystalline. 

 Along the cleavages seams of quartz and calcite have frequently formed. 



Along the upper river section the rocks of the area are distinctly more 

 crystalline, and are chiefly biotite-schists and biotite-hornblende-schists, the 

 latter often very coarse. They are sometimes banded, but very irregularly, 

 the lenticular character of the banding suggesting the rhombic cleavages 

 of the southern section. Some of the finer-grained biotite-schists contain 

 round or elongated areas of quartz and epidote, which resemble amygdules. 

 With these are associated considerable thicknesses of sericite-schists, full of 

 little eyes of blue quartz; these are evidently metamorphic acid eruptives. 

 The width of the northern section is about 2,000 feet. 



Under the microscope the Hemlock schists of the Fence River area 

 have a general porphyritic habit. Two main divisions only are clearly 

 distinguished. One of these is the fine-grained mica (sericite) schists, 

 which are characterized by the presence of muscovite as well as biotite in 

 the microcrystalline groundmass, and true phenocrysts of feldspar and 

 bipyramidal quartz, while the other embraces all the other varieties, which, 

 diverse as they undoubtedly are, have yet certain important characters in 

 common and are connected by gradations. The sericite-schists are obviously 

 metamorphosed acid lavas, and need not be described in detail here. 



The origin of the second division, however, is far more obscure. The 

 least altered of these rocks possess an exceedingly fine grained microcrys- 

 talline groundmass, made up of very pale chlorite and a colorless aggregate 



