SoLLAs — On the Structure and Origin of Quartzife Rocks. 175 



sufficiently different to that of quartz to render the latter very- 

 conspicuous. Sometimes the sericite is replaced by a layer of 

 white opaque earthy matter which by transmitted light appears 

 as a black line round the quartz grains. The quartz is colourless 

 and transparent, like much of that which enters into the compo- 

 sition of the granites of the district, from which, however, it 

 cannot have been derived, as these are of later age. The outlines 

 are more or less irregular, but at the same time usually rounded : 

 very many of the grains are oval or even approximately cir- 

 cular in section, and this sometimes when the grain is so minute 

 as to measure no more than 0'13 mm. in width by O'll mm. in 

 breadth (Plate xv., fig. 2) : no one accustomed to the microscopic 

 examination of rock-slices could hesitate for a moment as to their 

 clastic nature. The rounding of the grains is positive evidence 

 of great value, the irregular outlines which sometimes occur are 

 not in contradiction to it, since irregular and even angular out- 

 lines are common in grits of acknowledged sedimentary origin, 

 e. g. in the Old Red Sandstone grit of Kiltorcau, of which I give 

 an illustrative sketch (x 30 diameters) below (fig. 1). 



\ c0^' '^ 



Fig. 1. — Section of Grit from Kiltorcan [camera lucida tracing). 



If, while viewing a slice of the quartzite from Carrickgologan 

 (Plate XV., fig. 2), the analyzer be pushed in, its appearance under- 

 goes a singular transformation (Plate xv., fig. 1),^ the rock no 



^ The figures 1 and 2 (Plate xv.) are reproductions of photographs of the same field 

 of view — fig. 2 as seen by ordinary, and fig. 1 by polarized light. They are placed 



