STRATIGRAPHICAL ELEMENTS: CRYSTALLINES, ETC. 69 



cases it would seem as if individual crystals of the felspar of the 

 gneissose-granite had migrated from the intruding rock into the 

 intruded schist. Some examples of the more intense form of this per- 

 meation or injection of the schists by the gneissose-granite will be 

 given later, along with the microscopical details. The Shal N., Black 

 Mountain, exhibits numerous examples of this structure. 



I now proceed to a microscopical description of a few typical slices 



of the gneissose-granite in its various modifi- 

 Microscopical exam- . ' __ 



ination of slices of cations. No. T | T , half a mile north of Mansehruh. 



In the hand-specimen it shews a granite texture 

 with only slight parallelism of the component minerals, and is slightly 

 porphyritic, the felspars having all the appearance of being orthoclase, 

 as occasional binary twinned porphyritic crystals, 1 to 2 inches long, 

 seem to indicate. Nevertheless, under the microscope, much of the 

 felspar thus named comes out with a very distinct microcline-like 

 structure, others shew a parallel striped structure resembling ordinary 

 albite twins of plagioclase. Some of this characterising the smaller 

 felspars appears certainly to indicate plagioclase. Most probably the 

 microcline and striped appearances are pressure-induced structures. 

 The triclinic felspar is mostly idiomorphic with sharp outlines, whilst 

 the porphyritic orthoclase or microcline crystals shew a considerably 

 worn outline, as if the molten magma surrounding them had slightly 

 re-fused them. Considerable saussurisation of the felspars has taken 

 place in patches within the larger crystals. Within the larger por- 

 phyritic crystals of felspar there is also an appearance as of included 

 smaller crystals of felspar which are completely altered into a 

 grey dusty decomposition product, also a few quartz granules and 

 brown mica plates. In the rock-mass in the field these features can 

 often be seen in the larger 5-6 inch crystals of felspar. Quartz is 

 visible in the slice in considerable quantity, filling in the spaces 

 between the other minerals. The margins of the quartz-grains, and 

 the differently polarising polysynthetic mosaic or tesselated structure 

 into which it breaks up between crossed nicols, shew much powdering 

 up of the edges of the grains, although the rock is not typically 



( 69 ) 



