J. Parkimon — The Making of Quartz Schist. 261 



The remnants of original mineral grains are conspicuous in most 

 of the thin sections. The presence of felspar, both orthoclase and 

 plagioclase, together with quartz, indicate either a quartz felspar 

 grit or a quartz felsite. The great preponderance of quartz and the 

 very clastic appearance of the felspars clearly point to the former 

 rock ; in fact, in many instances the names of quartzite and quartz 

 felspar grit are still applicable, in spite of considerable mineralogical 

 and structural modifications. 



The condition of the felpars is the most interesting feature. 

 Mineralogical change is made manifest by a cloudiness in the interior 

 of the crystal, due to the presence of innumerable minute scales, 

 flakes, or prisms of a mineral which, no doubt, is a mica. Towards 

 the periphery of the crystal the latter pass into a fringe of well- 

 formed flakes intermingled with grains of quartz, the character 

 of the former being identical with that of the typical quartz 

 schists of the Alps. Occasionally a system of lines of greater 

 change, and probably of incipient fracture, traverses the crystal, and 

 is marked by closely aggregated scales and flakes of mica. At the 

 same time crush shadows form, followed in due course by fracture 

 and loss of uniform polarization ; while, frequently, more or less 

 clear spots appear representing developing individuals of quartz or 

 secondary felspar ; the outcome being complete disintegration and 

 reconstitutiou. 



Some of the larger quartz crystals have exceedingly irregular 

 edges and exhibit undulose extinction ; and in the smaller grains 

 a similarity of size and of polarization tints is often characteristic, 

 together with some elongation in the direction of foliation. In 

 a more advanced stage of reconstruction a more marked parallelism 

 exists between the flakes of mica. While these are for the most 

 part spread through the rock in thin and irregular bands, they are 

 also embedded in the quartz grains, or wedged between their edges, 

 and have even trespassed the boundary between two individuals. 

 In some sections they reach -009 inch in length. The presence of 

 a mica flake in the centre of a quartz gi-ain, which is large in 

 proportion to it, and situated near a system of fine cracks by which 

 this grain is traversed, indicates the passage of the mica substance 

 in solution by capillary action. The cracks contain specks of a 

 mineral (about "0025 inch across) which may be referred to mica. 

 Now and then such flakes appear where no obvious cracks exist, 

 and occasionally a long thin wedge of mica flakes penetrates the 

 quartz grain. The condition of the mica is different to that of 

 the mineral produced by the mere crushing of a quartz felspar grit ; 

 the distinction consisting in this, that in the rocks under consider- 

 ation the mica forms definite flakes usually bounded by definite 

 lines, whereas simple crushing produces a matted intergrowth of 

 film-like scales which ordinary powers and ordinary thin sections 

 fail to resolve into its components. The smaller quartz grains — 

 commonly -004 to '009 inch across — possess angular or subangular 

 outlines, are not conspicuously elongated at right angles to the 

 direction of pressure, and do not exhibit undulose extinction to any 



