RESEARCHES IN THERMAL METAMORPHISM. 189 



The Ordovician and Silurian rocks of the district have 

 a great variety of characters. The clay-slates pass through 

 the stages of " Knotenschiefer," " Knotenglimmerschiefer," 

 and " Hornfels " with the production of biotite, quartz, mus- 

 covite, etc., but cordierite is uniformly produced instead of 

 andalusite. Pyrite is replaced by pyrrhotite. The rutile- 

 microlites of the original slates are recrystallised as larger 

 grains of rutile or locally converted into little crystals of 

 brookite. The siliceous rocks (" Kieselschiefer," etc.), some 

 of which seem to be radiolarian cherts, are converted into 

 quartzites ; graphite, up to as much as two per cent., repre- 

 senting the original carbonaceous matter. Other minerals 

 produced are biotite, chiastolite, and pyrrhotite. The grey- 

 wackes are recrystallised as a mixture of quartz, biotite, 

 muscovite, plagioclase, etc., while their clastic origin is still 

 indicated by angular fragments of quartz which keep their 

 original outline, but are seen to resolve in polarised light 

 into a mosaic of smaller grains. 



The calcareous rocks give rise to an important observa- 

 tion which is probably of general application. The pure 

 limestones are simply recrystallised as marble, while the 

 impure ones lose their carbonic acid, and pass into rocks 

 consisting mainly of lime-bearing silicates. The charac- 

 teristic minerals here are garnet and augite : actinolite, epi- 

 dote, zoisite, etc., also occur, but wollastonite, idocrase and 

 scapolite are not found. The absence of wollastonite seems 

 rather remarkable, since the limestones analysed are not 

 magnesian. In other places more complex results have 

 been produced by an impregnation of the limestones with 

 ore-material, probably concurrent with the metamorphism. 



Certain "diabases" (apparently andesitic lavas) show 

 considerable metamorphism, the most characteristic feature 

 being the amphibolisation or uralitisation of the augite, 

 which process is seen in various stages. Besides the light 

 green hornblende, which is the most prominent constituent 

 of the altered rocks, occur subordinate biotite, unchanged 

 augite, sphene, iron-ores, etc., while the general ground- 

 mass of the rock is a mosaic of clear granules of plagioclase. 

 The characteristic pyrrhotite is here wanting. Tuffs asso- 



