382 B. N. PEACH AND J. HORNE ON THE 



a crystal of orthoclasc displays the Carlsbad type of twinning, polarising with 

 different colours on different sides of the median line. The quartz occurs in 

 grains and in crystals, clear and colourless, with numerous fluid inclusions. A 

 small quantity of apatite is present in the section. There is no trace of any 

 felsitic ground mass, and it is therefore evident that the rock most nearly 

 approaches the character of a binary granite or aplite. Another section from 

 the north bank of Rooeness Voe exhibits the same coarsely crystalline charac- 

 ter as the preceding example ; the felspar predominating over the quartz. 

 Besides the orthoclase, which is also kaolinised, there is some microcline which, 

 with polarised light, displays that peculiar rectangular arrangement of the 

 striations traversing the mineral. Magnetite is also present, merging in some 

 instances by hydration into limonite. The microscopic examination of sections 

 taken from various localities throughout this mass only confirms the conclusion 

 previously arrived at by Hibbert and Dr Heddle. The rock is essentially 

 granitic, but owing to the singular absence of mica over the greater portion of 

 the sheet, it is different from the older granites in Shetland, represented by 

 the masses at Dunrossness and Bixetter Voe. 



A section taken from near the margin of the Sandsting sheet in Gruting 

 Voe contains much orthoclase which has been considerably kaolinised, large 

 grains of quartz, and a little black mica. In short, it is a typical granite. 

 Another specimen from the hills west of Skeld closely resembles the foregoing, 

 with the addition of a small quantity of microcline. At Laxa Burn the rock 

 consists of an admixture of red orthoclase and quartz. 



The rock occurring near the diabase on Skeld Hill shows, under the micro- 

 scope, prisms of orthoclase and plagioclase ; the latter predominating, and less 

 altered than the former. With these are associated clear quartz and dark 

 mica, which are strongly dichroic. Needles and prisms of apatite occur abun- 

 dantly in the section, while magnetite is also present along with some crystals 

 of sphene. A specimen, not far removed from the foregoing, exhibits a fair 

 proportion of green hornblende, which is dichroic, along with biotite, the other 

 constituents being the same. In the sections prepared from the Sandsting 

 sheet no trace of a felsitic ground mass has been observed. The only note- 

 worthy microscopic difference between the Sandsting and Rooeness sheets is 

 in the development of mica and hornblende in the former mass. 



Far more interesting and exceptional are the microscopic characters of the 

 Papa Stour sheet of pink or salmon-coloured felsite. Instead of presenting a 

 marked crystalline aspect, it shows a felsitic ground mass of a reddish-brown 

 colour, in which few felspar crystals are discernible. This ground mass is 

 highly decomposed, and hence, even with a high power, it is impossible to 

 define the fine granular constituent, as the base has little action on polarised 

 light. The characteristic feature of the rock is the well-marked spherulitic 



