342 LIETJT.-GEN-. C. A. MCMAHON ON [May 19OO, 



mosaic, in which some of the felspar is occasionally involved. This 

 mosaic at times behaves after the manner of an intruder cutting 

 across quartz and felspar, like granite permeating and penetrating 

 sedimentary rocks. Connected with this phenomenon is the 

 frequent occurrence of granophyre, and the intense erosion and 

 perforation of biotite, felspar, and other minerals by the quartz of 

 the granite. (See PL XXIII, fig. 4.) 



Another structure met with in the Hatu Pir Granite may be 

 noticed in passing. Sometimes the quartz, instead of breaking 

 up into granulitic, tessellated, or more or less rounded granules, 

 assumes very irregular and varied outlines : the individual granules 

 throwing out finger-like processes, and dovetailing one into the 

 other in a very complex way. Fig. 1 (PI. XXIII), photographed 

 from one of the slides, illustrates this structure. In the following 

 pages I shall speak of it as the amoeboid structure. Like the 

 tessellated structure (of which it is only a variety), it indicates 

 that final crystallization set in under conditions of great strain. 



The Hatu Pir Granite reminds me strongly of the gneissose 

 granite of Dalhousie, and, like the latter, it is rich in microcline. 



(3) The Acid Variety of the Hatu Pir Granite. 



The Hatu Pir Granite is cut through by another biotite-granite, 

 so closely resembling it that I regard it as an acid variety of the 

 same rock belonging to a later phase of the eruption. 



Alineralogically considered, the acid variety is composed of the 

 same minerals as the normal Hatu Pir Granite, namely, quartz, 

 orthoclase, oligoclase, a little andesine, biotite, a little muscovite 

 or silvery mica, 1 magnetite, epidote, sphene, zircon, allanite, and a 

 very little schorl. 



The granite appears to have emanated from the Hatu Pir magma 

 after the relative proportions of the constituents had been some- 

 what altered by the crystallizing-out of some of the basic 

 material. In the Hatu Pir Granite free quartz falls below felspar 

 in amount in nine cases out of ten, and equals it in the remaining 

 case. In the variety uuder description the quartz falls below felspar 

 in four cases only, equals it in one, and exceeds it in four cases 

 out of a total of nine. These figures seem to show an increase in 

 the proportion of silica. 



A diminution in the proportion of alkali present seems also to 

 have taken place. In the Hatu Pir Granite orthoclase predominates 

 over plagioclase in eight out of ten cases ; whereas in the acid 

 variety it does so in five only out of nine cases. 2 



A diminution is observable in some of the more basic minerals. 

 Biotite, magnetite, and apatite are, generally speaking, less abundant 

 than in the parent-rock, and sphene only occurs in one slice. 



1 The muscovite I regard as an original, and the silvery mica as a secondary 

 mineral. 



2 The percentage of alkali is considerably higher in orthoclase than in 

 plagioclase. 



