PETHOGEAPHIGAL CHARACTERS OF HEMLOCK FORMATION. 445 



the suiTounding' zones. It is probable that many of these nuclei represent 

 an early generation of epidote, like the small irregular grains of the ground- 

 mass, -^hich were subsequently enlarged to porphyritic size. Inclosures of 

 zoisite are not uncommon in the large epidote individuals. Large lenticular 

 aggregates of epidote with calcite, chlorite, and biotite are found partially 

 replacing feldspar individuals, which were no doubt original phenocrysts. 

 Similar aggregates unmixed with the remains of feldspar are not infrequent, 

 and may reasonably be attributed to the same source. Epidote with quartz 

 is also the common filling of the amygdaloidal cavities. 



Common hornblende, actinolite, and ottrelite are very common as 

 porphyritic constituents of the schists. Hornblende occurs in very lai'ge 

 well-formed single crystals and clusters placed at random through the 

 groundmass. It is characteristically associated with ottrelite and biotite, 

 and often has formed somewhat later than the latter. It is always crowded 

 with inclusions, which in the laminated varieties carry the structure through 

 without reference to the position of the host. Ottrelite is abundant in some 

 of the sections, and is distinguished by its characteristic pleochroism. It 

 occurs in large individuals and multiple twins, and like the large horn- 

 blendes and biotites is full of inclusions. 



The general characteristics of these schists then are, first, a groundmass 

 composed of chlorite, quartz, magnetite, epidote, and in some cases contain- 

 ing plagioclase microlites, and secondly the presence in this groundmass of 

 much larger porphyritic individuals of several secondary minerals. The 

 varieties are determined by the varying ratio of the porphyritic constituents 

 to the groundmass, by the nature of the predominant secondary minerals, 

 and also by the differences in grain of the groundmass. This, while gen- 

 erally extremely fine grained is much coarser, but without minei-alogical 

 change, on the northern river section where the schists are more distinctly 

 crystalline. The cleavage of the schists is determined by the arrangement 

 of the minute particles of the groundmass, and not by the parallelism of 

 the lai'ge secondary minerals. These last, further, are never faulted or 

 broken, and in general are unstrained optically. They must have formed 

 then after the compression and tilting of the series. 



The origin of these schists, I think, is not doubtful. As important 

 points of evidence we have, first, the absence of rocks possessing any sedi- 

 mentary characters throughout the whole section. Next we have the 



