lOU Mil. C. E. TILLEY 0>' THE GEAyiTE-G->'EISSES ;_yol. IxXTii, 



its rarity in the amphibolites under discussion is noteworthv, 

 pointing to an original pyroxene, fi'oni which the hornblende is 

 ultimately derived, of low titanium content. Quartz is usually 

 present in small amount, and this is to be regarded as a normal 

 feature of amphibolites Avhich have been derived by metamorphism 

 of basic igneous rocks. Biotite and pyrites are further accessories. 

 A small amount of biotite developed in flakes in the amphibolite 

 imparts to the rocks a distinctly foliated appearance. 



Quartz and orthoclase are essential constituents of certain 

 amphibolites, and these two minerals may in such cases be present 

 in considerable amount. These constituents, therefore, mark off a 

 dividing-line fi'om the amphibolites which have already been 

 discussed. 



Quartz- and Orthoelase-bearing Amphibolites. 



Amphibolites in which these m.iiierals are conspicuous are not 

 characterized by any definite texture. Some of them are com- 

 paratively coarse-grained, are foliated like hornblende-schists, and 

 have been injected lit-par-lit fashion by the invading gneisses. 

 There are others of quite massive appearance, and of very fine 

 grain, not unlike that of a dense basalt. 



These types are to be met with principally in the Lincoln area. 

 A m.edium- grained inclusion at Kirton Point, showing veining and 

 intrusion by the acid gneiss, consists of hornblende, plagioclase^ 

 orthoclase, quartz, biotite, apatite, and magnetite. While the 

 predominant minerals are hornblende and plagioclase, yet the 

 potash-felspar and quartz are not inconsiderable constituents. 

 The orthoclase is readily picked out by its low refractive index 

 and small extinction-angle. The quartz is usually distinguished 

 from the plagioclase by its higher double ]-efraction, and by 

 absence of t^vinning. when this is a constant character of the 

 plagioclase. A useful point of discrimination is the characteristic 

 undulose extinction developed in bands in the quartz, which is 

 seldom to be observed in the felspar. The amount of quartz in 

 some of these types is clearly in excess of that postulated by any 

 conversion of pyroxene to amphibole. The composition of the 

 plagioclase in the rock under discussion points to andesine < Ab^An.J, 

 giving an extinction-angle of 20^ in sections perpendicular to the 

 ciystallographic axis a. The apatite is present, both as fine 

 acicular needles and as rounded s:rains of larsfer size. 



As an example of a fine-grained amphibohte which is an ortho- 

 elase-bearing tvpe, a band developed in the gneiss on the east side 

 of Point Boston may be taken for description. This rock affords 

 one of the best instances of the therm.al effect of the engulfing 

 gneiss on the amphibolites. The band has been rendered plastic 

 by its incorporation, and drawn out to follow the intiicacies of the 

 flowing gneiss. It is a dark-grey rock, of dense structure, and has 

 a specific gravity of 2'95. The constituents, as seen under the 

 microscope, are plagioclase, hornblende, quartz, orthoclase, magne- 

 tite, apatite, and a trace of pyi'ites in cubes. A foliated appearance 

 is given by the parallel orientation of the hornblende-prisms. 



