26 abstracts: geology 



there occurred such a chemical recombination of the silica, alumina, 

 potash, iron and water of the feldspar, kaolin and iron of the shale, as to 

 generate new potash mica in amount sufficient to constitute, in the 

 mica slates, over 33 per cent of the resulting slate. This muscovite 

 was of infinitesimal thinness and other dimensions and mostly of longish, 

 tapering or ribbonlike outline. Most of these scales arranged themselves 

 with their flat sides parallel to, or overlapping, one another but facing 

 the direction from which the pressure came and also with an angle of 

 inclination governed by that pressure. A small but variable proportion, 

 however, of these scales took such a position that their flat sides be- 

 came parallel to the direction of the pressure thus giving rise to the 

 "grain." 



As mica crystallizes in columnar crystals and as the plates or scales 

 due to its molecular structure are transverse to the crystal column, 

 and as a slab of slate consists largely of parallel scales of mica it may be 

 said to correspond, when held horizontally, to such a crystal held ver- 

 tically. When a mica slate is cut in thin section across the cleavage 

 its optical behavior under polarized light is like that of a mica crystal 

 cut across its crystal cleavage. Yet as not only a considerable num- 

 ber of the mica scales in slate lie across the cleavage, but as some scales 

 of chlorite and crystals of other minerals do also, the texture of a mica 

 slate combines some of the features of a crystal with some of those of a 

 tissue. 



This crystalline fabric may inclose in its meshes any sedimentary 

 particles of quartz, zircon, feldspar, kaolin, or other minerals which were 

 not or could not be made over into mica or secondary quartz but 

 whose alignment became more or less parallel to that of the major 

 part of the new mica. During this metamorphism other chemical 

 combinations were formed by the constituents of the shale, result- 

 ing in isolated scales or crystals of chlorite, biotite, various carbon- 

 ates, pyrite, magnetite, graphite, tourmaline, andalusite, etc. These 

 arranged themselves variously — some in the cleavage direction, some 

 in the grain direction. Lenses consisting of one mineral surrounded 

 by one or two others were also formed and concentrically or radiately 

 arranged. The subjects of slip cleavage, shear-zones, cleavage band- 

 ing, slate discoloration and weathering are considered in some detail. 



T. N. D. 



