320 



MESSRS. A. UARKKR AND J. E. MARR ON 



outer ring of metamorphism (KnotentJionschiefer) in some districts 

 of argillaceous strata, such as llosenbusch's * JSteiger Schiefer. The 

 spots in our rocks are comparatively free from coloured constituents, 

 and do not make their api)earance until after considerable develop- 

 ment of secondary minerals. The original pigment of organic 

 matter is dissipated as the first result of metamorphism. More- 

 over, the spots in the Brathay Flags, when best developed, show a 

 distinctly crystalline structure between crossed nicols, being evi- 

 dently imperfect crystals charged with a large quantity of foreign 

 inclusions. The ovoid form seems to be that of imperfectly formed 

 crystals, for the general ground of each spot extinguishes parallel to 

 the long axis of the irregular oval, which has no universal direction, 

 but lies quite at random. Possibly the mineral may be andalusite. 



Apart from the spots, it will be noticed that andalusite is absent, 

 as well as other characteristic aluminous " contact-minerals." It 

 would appear that the rock contained sufficient alkalies to build up 

 a large part of the alumina present into secondary felspars. 



The Acins of white mica have already been mentioned. The 

 rock adjacent to these shows some curious modifications, being 

 built in great measure of a clear colourless mica similar to that 

 occupying the veins [949, 1080J. This mica is partly in minutely- 

 matted aggregates, but mostly in well-defined flakes with rough 

 parallelism, moulded by a clear crystalline mass of grains, some of 

 which show felspar-twinning. The brilliantly polarizing flakes are 

 also moulded by another micaceous-looking mineral with a very pale 

 greenish-grey colour and feeble dichroism, but not sensibly bi- 

 refringent. This may be one of the ripidolite group, but we have 

 not established its identity with any described variety. A little 

 magnetite and granules of the supposed pyrrhotite occur. Brown 

 mica is only sparingly associated with the white in this marginal 

 modification of the rock. At about an inch from the actual vein, 

 however, the spotted character of the rock is apparent, and brown 

 mica occurs as usual in the interspaces between the spots. There 

 is still a considerable amount of white mica, mostly in exceedingly 

 minute scales within the spots, but parth^ in more conspicuous 

 flakes near their margin. One slide [949] shows a crack running 

 at right angles to the main vein, its course marked by a slightly 

 coarser aggregate of colourless mica and quartz, with some clear 

 felspar, a little of the ripidolite-like mineral, and occasional grains 

 of yellow-brown tourmaline, touched here and there with blue. 



Of the remaining members of the Silurian formation we have 

 made no systematic examination, but the few specimens studied 

 offer some points worth recording. The Upper Coldwell beds and 

 the less calcareous portion of the Middle Coldwell, viz. the lowest 

 strata of that division exposed at Backhouse Hill, bear a general 

 resemblance to the uppermost beds of the Brathay Flags at the 

 same locality, and the resemblance is borne out by the microscope. 

 Numerous minute shreds of a mineral like tremolite disseminated 

 through these metamorphosed flags perhaps point to a certain 

 * 'Abb. zur geol. Specialk. v. Elsass-Lothr.' vol. i. (1877) part 2. 



