RESEARCHES IN THERMAL METAMORPHISM. 197 



former shows in thin slices a uniform mosaic structure. 

 Local conversion of sandstones into quartzites of this type 

 is frequently seen where the strata abut upon any consider- 

 able body of intrusive rock. In many cases the metamor- 

 phism is further made evident by the production of special 

 accessory minerals arising from impurities (decomposition- 

 products, etc.) in the original sandstone. Near the Shap 

 granite (3) the grits in the Coniston Flag group are con- 

 verted into quartzites containing numerous granules of 

 pyroxene, probably formed from kaolin and calcite in the 

 strata metamorphosed. The quartzite of Clova, in Forfar- 

 shire (2), contains a curious green mica, and other accessory 

 minerals are found in other examples. 



An interesting account has been given by Home (12) 

 ■of the changes produced in the radiolarian chert in the 

 Arenig beds of the south of Scotland near the large granite 

 mass of Loch Doon. At about a mile from the contact the 

 chert begins to take on a granulitic structure, which be- 

 ■comes more pronounced and coarser, while brown mica 

 begins to be developed. At half a mile from the granite, 

 where this change is well-marked, the traces of radiolaria 

 •can still be detected. Near the contact the chert is com- 

 pletely recrystallised, a curious feature being the numerous 

 perfectly rounded inclusions of biotite contained in the 

 quartz-grains, which otherwise constitute the whole of the 

 metamorphosed rock. The carbonaceous matter, which in 

 the analogous rocks described by Beck gives rise to graphite, 

 is here absent. In the Devonian and Carboniferous 

 " Kieselschiefer," metamorphism by the Brocken granite 

 has apparently expelled the carbonaceous matter almost 

 ■completely (Lossen, 1888). 



More impure gritty rocks give rise by metamorphism to 

 less simple products. Silicates of alumina, garnet, micas, 

 etc., may be extensively produced, and the resemblance to 

 a quartzite is lost. Sillimanite is very plentiful in some 

 highly metamorphosed rocks, such as those of Guemene in 

 Brittany, described by Barrois (1884). In coarse sediments 

 •of composite nature evidence of their clastic origin may be 

 preserved after very considerable modification, as we have 



