204 TEANSACTIONS OF THE [APR. 24 



Under the microscope the gneiss shows a mineral composi- 

 tion seldom very difterent from that of the granites, though, as 

 a rule, containing more plagioclase. Slides from difierent 

 localities show no great variation in the character of the 

 minerals present, tliough different proportions prevail. Even 

 the dark bands are usuall}' composed of the same minerals as 

 the normal gneiss, but with a larger content of mica or horn- 

 blende. There are, however, excejitions to this rule. Quartz, 

 orthoclase, plagioclase, biotite and hornblende make up the 

 bulk of the gneiss ; with zircon, ajjatite, ilmenite, magnetite, 

 rutile, garnet and sillimanite as minor constituents. 



Quartz is never absent, though varying somewhat in quantity. 

 It often occurs in rather large patches, which under crossed 

 nicols break up into several distinct parts. Fluid inclusions 

 are very abundant, usually in bands which extend through 

 adjacent individuals. Zircon is also a common inclusion, while 

 the hair-like bodies supposed to be rutile are rarely present. 

 The quartz nearly always shows undulatory extinction, wlncL, 

 together with the shattering often apparent, bears witness to 

 the strains to which the rock has been subjected. 



The feldspars show much variation in the relative propor- 

 tions of different species present. On the whole, plagioclase 

 seems rather more abundant than orthoclase, but the reverse is 

 so often true that the examination of a larger number of 

 sections might show it to be the rule. In a few instances 

 microcline is an abundant constituent, but it is more generally 

 absent. A very marked feature in a majority of the sections 

 examined is a great abundance of the microperthitic iater- 

 growth of orthoclase and plagioclase. The peculiar banded, 

 dotted and striated sections produced by it are generally 

 apparent in ordinary light, but are brought out more clearly 

 with crossed nicols. Sections of orthoclase show a varying 

 proportion of their ai'ea occupied by slender spindle-shaped 

 masses of plagioclase. These spindles extinguish simultan- 

 eously, and have decidedly stronger double refraction than the 

 surrounding orthoclase. In most instances the microperthite 

 has the appearance of a contemporaneous crystallization of two 

 feldspars ; but enough sections contaiu absolute proof of its 

 secondar}'^ nature to render it extremely probable that in this 

 gneiss it is never an original intergrowth. Evidence of this 

 secondary origin is seen in plagioclase spindles passing 

 unbroken across cracks in the orthoclase, and in the evident 

 optical continuity of the material of the spindles with secondary 

 feldspar filling cavities and cracks adjacent to the microperthite. 

 Figure 1 shows a case of the latter kind. A crack between 



