THE SHAP GRANITE AND ASSOCIATED KOCKS. 319 



rest is very obscure. The rather irregular spots, -J,j to -jj\y inch in 

 diiimcter, are difFerentiated by their comparative freedom from mica 

 [1219]. 



At 3G0 yards from the granite the spots are more regularly ovoid 

 and their boundary more sharply defined, the brown mica in the 

 interspaces forming distinct small flakes arranged tangentially to 

 the outlines. The central part of each spot contains smaller flakes, 

 often rather rounded, but the marginal zone is free from mica 

 [1218]. (See PL XII. fig. 5). In some specimens from this neigh- 

 bourhood the mica in the general body of the rock has a marked 

 parallel arrangement, which corresponds to the lamination of the 

 original flags [864]. 



An example from Packhouse Hill has less mica, and that of a 

 pale colour, but here pyrrhotite is exceptionally plentiful, and has 

 presumably used up most of the iron which has elsewhere gone into 

 the usual brown pleochroic mica [1222]. This is at 600 yards 

 from the granite. A specimen from Collyrag Quarry, a hundred 

 yards nearer, shows similar characters [1079]. These rocks are on 

 a slightly higher horizon than the preceding. They show little or 

 no indication of " spots," have rather abundant clastic quartz, and 

 present a considerable resemblance to the Upper Coldwell beds 

 exposed farther south. 



The normal brown mica of the metamorphosed flags resembles in 

 general characters that which has been produced in the andesitic and 

 other rocks described above. Such mica has a special quality as seen 

 in reflected light, which gives a peculiar purplish-brown sheen to the 

 rocks in which it is abundant. With this goes a very intense pleo- 

 chroism in thin sections, the absorption being almost complete for 

 vibrations parallel to the cleavage-traces, while, if the nicol be turned 

 a very little away from this position, a distinctly greenish-brown 

 colour is seen. Similar characters have been described by various 

 writers in the mica of " contact '"' rocks in other districts, and it would 

 be interesting to ascertain whether the mineral is chemically difi:'erent 

 from the brown micas of igneous rocks. The only investigation we 

 can find on this point is in Lang's brilliant paper on the Christiania 

 district, already referred to above. He and Jannasch separated and 

 carefully analysed the brown mica of a Glimmerliornfels in that 

 district. They found it to contain 7*98 per cent, of magnesia and 

 21*94 of ferrous oxide, ferric oxide being entirely absent: also 

 titanic acid occurs to the extent of 3'40 per cent *. Except for the 

 absence of ferric oxide the figures differ but little from Schliipfer's 

 analysis of the biotite of Miask. The last-named author has shown 

 that the earlier analyses of micas leave much to be desired in point 

 of accuracy. 



The nature of the " spots " in such rocks as these is not an 

 easy question, and it seems clear from the literature of the subject 

 that the phenomena of spotted and knotted slate-rocks arise in 

 several different ways. AVe find nothing of the local accumulation 

 of the " pigment " of the rock into spots, which characterizes the 



* Op. cit. p. 318. 



