1889.] Contact Metamorphism in Silurian Rocks. 



199 



Scotland, we are at once struck, notwithstanding the present crystal- 

 line condition of the rocks, with their resemblance to those cut from 

 greywackes, which contain distinct fragments of quartz, felspar, and 

 an argillaceous rock. We do not, indeed, observe an indubitable 

 fragmental structure, but what we might term the ghost of one. For 

 instance, the eye is at once arrested by subangular grains of quartz, 

 which are much larger than those helping to form the mosaic of 

 quartz and mica which constitutes the major part of the rock. Their 

 edges are sometimes sharply limited by flakes of brown mica and are 

 rectilinear, at others they join irregularly to the quartzes of the 

 matrix, as if they had grown together. Sometimes these larger 

 grains show a compound structure, but more often they are simple, 

 part of a single crystal, just as may be seen in the fragments common 

 in a greywacke. Usually they are fairly free from enclosures, though 

 occasionally microliths and small fluid cavities with bubbles may be 

 observed. Other patches, yet more shadowy in outline, consist of 

 chalcedonic or microerystalline quartz and flakelets of mica, generally 

 white. Still, it is generally not difficult to distinguish them from 

 the matrix, owing to their difference in composition and structure. 

 Careful examination of these detects occasionally less altered portions 

 in which the lamellar twinning of a plagioclase felspar is still dis- 

 tinctly recognisable. These, then, represent the felspar fragments of 

 the original greywacke. Again we note other subangular fragments, 

 which consist sometimes of fair-sized quartz-granules together with a 

 little dark-coloured mica, or perhaps an iron oxide, sometimes of 

 smaller quartz-granules and of brown mica. Comparing these with 

 specimens of fine-grained earthy sandstones, silts, and shales, affected 

 by contact metamorphism, which we have obtained from other 

 localities,* we find a perfect correspondence in composition and struc- 

 ture, and have no hesitation in regarding them as representatives of 

 bits of arenaceous and argillaceous rock, once present in the original 

 greywacke. Fragments of granitoid or of volcanic rock have not 

 been positively identified in the present set of slides, though very 

 probably the former, at any rate, would be found, if a larger series 

 was prepared. The matrix of the original rock, once a silt of variable 

 composition, is now replaced by quartz and mica (generally brown), 

 with sometimes a considerable proportion of a pyroxenic mineral, 

 which appears to substitute itself for the brown mica. 



In conclusion, a few remarks may be made upon the specimens. 

 The first group (see above, p. 194) was taken from near the granite 

 mass. All the specimens have a hard, strong appearance ; some show 

 a slight mineral banding, but there is no well-marked foliation ; thus 

 the texture of the rock at the first glance reminds one more of a 

 rather micaceous band in those granulitic rocks which are generally 

 * Bonney, 'Quart. Journ. Greol. Soc.,' toI. 41, p. 11. 



