88 MR. C. E. TILLET ON THE GEAISTITE- GNEISSES [vol. Ixxvii, 



the Archaean intrusives is considered, is a point of some significance.^ 

 It may well be that the Archa3an granites were intruded at greater 

 depths below the surface, or at least intruded into areas of higher 

 regional temperature with less steep temperature-gradients, so that 

 in the final stages of crj^stallization the residual solutions were able 

 to act for much longer periods on the alread}'- crystallized minerals. 

 The abundant presence of the high-pressure mineral, garnet, in 

 many of these older Archaean intrusives is in agreement with this 

 view. Myrmekitization is thus in part an albitization. 



On the other hand, there is conclusive proof that some myrmekite 

 ma}" be of metamorjDhic origin. Dr. J. J. Sederholm " has noted 

 the secondary development of myrmekite in granite which has been 

 subject to strong mechanical metamorphism, with the development 

 of mortar-structure. Here the mvrmekite has formed alono; the 

 zones of trituration, and can only be interpreted as posterior to this 

 * mortar ization.' The same features may also be observed in the 

 Lincoln area, where a fine secondar}^ myrmekite has arisen in a 

 similar fashion. The conditions prevailing at the conclusion of 

 this ' mortarization ' may well be not very different from those 

 prevailing at the latest stages of consolidation (see p. 98). 



In certain of the gneisses of Boston Island and the southern 

 coast of the hundred of Flinders, a pj^roxene takes the place of 

 hornblende or biotite, and sometimes to their complete exclusion. 

 This mineral is either a pale-green non-pleochroic diopside with 

 diallagic lamination, or more commonl}^ a pleochroic hypersthene, 

 the rock then conforming to the original charnockite tj^De. 



The later differentiation -products of the gneisses consist of a 

 series of hornblende-pegmatites and aplites. 



The aplites generally form dyke-like masses cutting across the 

 foliation, utilizing in this case the cross-jointing surfaces. Some 

 of the hornblende-pegmatites have a similar habit, but many 

 are in reality segregation-pegmatites. I have already noted the 

 distribution of these pegmatites with reference to the amphibolite 

 inclusions. They form a border-zone or halo to the amphibolites, 

 and fill in the spaces between the separated portions of one 

 amphibolite lenticle. It is clear that in some cases the amphibolite 

 has jdelded hornblende to the invading solutions. These hornblende- 

 pegmatites pass by an imperceptible gradation of texture into the 

 normal granite-gneisses. 



The constitution of the pegmatites is very similar to that of the 

 gneisses, both microcline and plagioclase (oligoclase-andesine or 

 andesine) being found. Some of this andesine shows acicular 

 inclusions similar to those observed in the felspar of the main 

 granite-mass. Biotite is a rare constituent of most of these 

 pegmatites. The hornblende is a black prismatic type, with well- 

 developed idiomorj^hic outline, attaining as much as 1| inches 



^ Myrmekite is present, however, in some of the Caledonian intrusions of 

 Scotland : for example, in the granites of Galloway. 

 ^ Bull. Comm. Geol. Finl. No. 48 (1916) pp. 127-29. 



