114 The Sedimentary, Metamorphic, 



these rocks, for it is very frequently crushed, broken, worn 

 at the edges, and generally showing the effects produced by 

 long subjection to heat, and also physical movements of the 

 mass in which it existed in a crystallised form. Fragments 

 of orthoclase also occur " jammed" into corners or included 

 in the residual quartz. 



The triclinic felspars are almost always in smaller and 

 better-formed, much-compounded crystals. 



It is characteristic of these gneissic schists to have two 

 kinds of mica. 



The earlier-formed one is a, magnesia-mica, much poorer 

 in iron than that of those gneisses which I have found else- 

 where in the district as margins to the massive intrusive 

 rocks. This mica of the gneissic schists is usually in the 

 foliations with the felspars, but is also to be found included 

 in the quartz. The second mica is a colourless alkali-mica, 

 such as I have already mentioned when speaking of the mica 

 schists, and I think that in many cases it is a secondary 

 production. It is very characteristic of these gneisses that 

 some of their constituent minerals have been altered to 

 pinite. Some of the felspars certainly have ; cordierite, also, 

 so far as one can judge from pseudomorphs, and perhaps in 

 the largest measure the magnesia-mica, which in an unaltered 

 state is still plentiful in some of the foliations. . 



Many of the gneissic schists are so massive that it is only 

 when they are examined in situ on the large scale that their 

 character as metamorphosed sediments, and not varieties of 

 igneous rocks, can be fully recognised. 



Aplite is the first of the igneous rocks which I have to 

 notice. Rosenbusch* defines aplite as a very fine-grained 

 rock composed of quartz, orthoclase, plagioclase, and potassa- 

 mica, Pegmatite includes the coarser-grained varieties. 

 He includes both under the section " Muscovite granites." 

 He notices as an exception the occurrence of magnesia-mica 

 in the aplites of Cornwall. 



My own observations in the Australian Alps show me 

 that there are here also instances of aplites which have 

 either magnesia-mica together with a potassa-mica or alone, 

 but in all cases the former is in very small amount. 



Besides the essential general characteristics of this rock, 

 — namely, a paucity, or even almost absence, of mica, with 



jPhysiographie der Massigen Gesteine, p. 19. 



