1889.] Contact Metamorphism in Silurian Bocks. 



201 



Much of this must once have been a more or less sandy argil] ite. 

 One or two, however, suggest the former presence of a fine-grained 

 quartz-felspar rock, probably a micro -gran ite, the felspar of which 

 has been replaced by a minute fibrous mineral, the fibrillse of which 

 exhibit bright tints when rotated between crossed Nicols, and give 

 straight extinction ; probably it is sillimanite. White mica is present 

 in rather considerable quantity, in thickish flakes up to about 0"04" 

 in longer diameter. Sillimanite is also present in the matrix, and in 

 parts of the slide a clear mineral, varying in colour from orange to 

 rich burnt-sienna brown, forms a kind of base to the small round 

 grains of quartz and flakes of brown mica. This material has but a 

 weak depolarising power. It exhibits an aggregate structure, and 

 appears to be in rather close relation with the above-named fibrous 

 mineral. It may be only a stained variety of the same, or possibly an 

 alumina- subsilicate nearly related to staurolite.* 



Proceeding onwards, the next specimen (415), taken about 

 10 yards from the junction, is macroscopically very distinctly speckled, 

 and under the microscope indicates very clearly the presence of frag- 

 ments. These are quartz and felspar as usual, an altered argillite, 

 and in one case an impure fine-grained sandstone or quartzose silt. 

 In the matrix is a considerable amount of hornblende. 



The next specimen (416), collected about 200 yards north of the 

 last, contains no hornblende, and is less definitely fragmental. The 

 flakes of brown mica are not seldom about 0"01" long, and occasionally 

 rather more. They indicate a slight foliation, and are frequently 

 crowded round the quartz grains, the latter being unusually clear 

 and free from enclosures. 



The next two specimens (417, 418) illustrate a rock with bands 

 of brownish and greenish-grey. Fragments are present, as above 

 described, some evidently have been fine-grained sandstones, a few 

 probably an argillite. The amount of these and the changes in the 

 matrix from the ordinary quartz-mica rock to layers where a 

 pyroxenic mineral replaces more or less the mica, indicate that the 

 original rock was composed of stratuhe of different nature, and a 

 slight foliation may be noted parallel with the original bedding. 

 Some of the pyroxenic mineral is hornblende, but in parts of the 

 slide an augite is abundant, as described above. 



The last specimen (419) from this locality gives less distinct indica- 

 tions of original fragments, and differs only from the ordinary quartz- 

 mica rocks in being rather more definitely foliated. 



The next two specimens (420, 421) were collected at Newton 

 Stewart Road, about £ mile from the granite, from beds which struck 



* A rather similar mineral occurs in a junction specimen from Sinen Grill, 

 Skiddaw, but here its structure is more uniform and its influence on polarised light 

 more marked. This very probably is staurolite, . . . 



