102 MR. C. E. TILLET ON THE GEANITE-GNEISSES [vol. Ixxvii, 



It bears perhaps most resemblance to the quartz-gabbro of 

 G-eorgia, only differing notably from this in the content of ferric 

 oxide. The iron content finds a parallel in the amphibolite recorded 

 by Becke, Analysis III. The diabases of New Jersey and Wash- 

 ington also resemble it in ultimate composition, the former of 

 these containing quartz as a modal constituent. 



An instructive illustration of the mode of development of an 

 orthoclase-bearing amphibolite is found in a prominent amphibolite- 

 band included in the gneisses at Kirton Point. The band in 

 question is strongly foliated at its border, and has been injected 

 lit-par-lit fashion by the pegmatitic solutions. At its centre, 

 however, it is free from any intrusive veins, and presents a much 

 less foliated appearance. 



A thin section of the border-rock near an invading pegmatite 

 veinlet shows it to contain hornblende, plagioclase of andesinic 

 composition, some orthoclase, and quartz evenly distributed. 

 Apatite and magnetite form the accessories. This veinlet of 

 pegmatite is one of the plagioclase types of hornblende-pegmatite. 

 It is of interest to note that, while the plagioclase of the amphi- 

 bolite is water-clear and undecomposed, the felspar of the vein is 

 largely filled with sericitic products. This selective weathering of 

 felspar of igneous origin as compared with m.etamorphic felspar 

 is referred to elsewhere (p. 110). 



When the central more massive portion of the band is examined 

 microscopically, an entirely different textural composition is seen. 

 It is in this central portion of the mass that a clue to the origin of 

 the amphibolite is given. The texture and composition is that 

 of an igneous rock. The constituent minerals are plagioclase, 

 pyroxene, hornblende, magnetite, apatite, quartz, and orthoclase. 



The plagioclase occurs in lath shape to prismatic form, and carries 

 many tine dusty enclosures, most of which are zonally arranged. 

 These felspars are the original unchanged crystals of the primary 

 igneous rock. The pyroxenes include both an augite and the 

 rhombic hypersthene, the latter in grains showing pink to green 

 pleochroism. The augite is a grey greenish type, and is the 

 dominant pyroxene. The hornblende is of the usual type, and has 

 been derived from the pyroxenes, as this uralitization can be seen 

 in progress. 



The quartz and orthoclase form rude intergrowths on a minute 

 scale of a somewhat granophyric type, between the remaining 

 m.inerals. It is quite clear that these intergrowths were the last 

 to consolidate, and the structure of certain granophyric quartz- 

 dolerites is here recalled, or in particular the micropegmatite 

 interstices of many dolerites. 



This rock is therefore a definite igneous type, in which the 

 uralitization of portion of the pja'oxene is the main impress of 

 m.etamorphism. When, however, the border of the same rock-mass 

 is examined, it is seen that the metamorphism involved has been of 

 a m.ore fundamental type. There is now no pyroxene remaining, 



