S2 COLEMAN ON THE MICEOSCOPIC 



prisms, and pyrite. Except iu porphyritic blebs of quartz, no fluid cavities were observed 

 in the halleflintas. These obscure rocks vary raiTch in composition, and transitional 

 forms connecting them with gneiss and felsite are easily found. Some specimens, con- 

 taining minute particles of hornblende in large numbers and but little felspar, are hard 

 to separate from the compact varieties of horublendic schist. 



Felspar Rock. 



A specimen of pale flesh-colored rock, consisting of plagioclase and probably ortho- 

 clase, with scarcely anything else, may be mentioned here, as the rock seems schistose. 

 The felspar individuals are ill defined and badly weathered to a granular substance of 

 doubtful nature. 



The other schistose rocks, such as mica schist, quartzite, etc., have not been observed 

 by me in the drift of this region. 



BASIC SERIES. 



MASSIVE. 



The rocks are characterised by the presence of a plagioclase other than microcline 

 and hornblende or augit«, almost invariably accompanied by magnetite or titanic iron ore. 



The massive rocks of the series may be divided into diorites, containing hornblende 

 or biotite as an essential mineral ; and diabases and gabbros, in which hornblende is 

 mainly or wholly replaced by some species of augite. 



Tlie Diorites. 



Most of these rocks are badly weathered, so that their constituents are not easy to 

 determine. Some specimens collected as diorites have proved to be diorite-schists on the 

 one hand, or diabases on the other, so that the group appears to be comparatively small. 



Macroscopic. — These rocks are in general medium grained, rarely coarse, and of a dark 

 green-grey color. 



Microscopic. — Quartz is found occasionally, but not in sufficient quantities to consti- 

 tute it an essential mineral. Secondary quartz is more common than primary. 



Felspar. — It is doubtful if orthoclase is present, but if so, only in small quantities, 

 and microcline has not been observed. On the other hand, the ordinary plagioclases 

 make up a considerable portion of the rock. They are usually turbid and much 

 weathered. The variety seems to vary much, the angle of extinction from the twin plane 

 ranging from 3° to 20". This would indicate oligoclase and labradorite with intermed- 

 iate varieties. Two systems of twin lamellce are commonly found, inclined to one 

 another at an angle of about 86°. 



In some of th^ felspars (probably plagioclase), the centre of the crystals differs from 



