RESEARCHES IN THERMAL METAMORPHISM. 297 



quartz and biotite, with scattered crystals of andalusite and 

 in certain layers blue grains of corundum. The intrusive 

 rock itself contains inclusions, some similar to the neighbour- 

 ing metamorphosed schists, others different and especially 

 rich in cordierite. 



More remarkable is the metamorphism of a lower group 

 of gneisses, mica-schists, etc. The minerals composing the 

 altered rocks are cordierite, andalusite, biotite, muscovite, 

 quartz, and felspars, with sometimes garnet, tourmaline, 

 sillimanite, spinel, corundum, and other minerals. Some of 

 these are original constituents of the rock, such as ordinary 

 orthoclase and muscovite, which disappear with further 

 metamorphism ; the rest are of secondary origin. In the 

 inner zone of alteration, extending 100 or 150 metres from 

 the contact, the rocks have lost all gneissic and schistose 

 structures. The dominant type contains 60 or 70 per cent, 

 of cordierite with biotite, andalusite, quartz, sillimanite, 

 ilmenite, zircon, etc. The tonalite itself has inclusions 

 which are taken to represent partially melted fragments of 

 the adjacent rocks. They are rich in cordierite, but contain 

 in addition spinel and corundum. 



Phenomena more or less resembling those described by 

 Salomon have been noted by Beck and Schalch in the 

 gneisses and mica-schists of Saxony (26). The metamor- 

 phism of mica-schists has also been described by Muller 

 from Schneekoppe, the highest mountain of the Riesenge- 

 birge, where the rocks abut upon an intrusive mass of 

 granite (27). Here andalusite is the most conspicuous new 

 mineral formed, in prisms several centimetres in length. 

 The filmy layers of pale potash-mica of the original schists 

 gives place to a dark magnesian variety in isolated flakes, 

 the quartz occurs in isolated grains instead of in lenticles, 

 and the schistose structure of the rock becomes much less 

 marked. Further, the imperfect crystals of garnet full of 

 inclusions are replaced by clean lustrous crystals with good 

 forms. The metamorphism here is clearly much less in- 

 tense than in the district surrounding the great tonalite 

 intrusion of the Adamello Mountains. 



It will be noticed that the most extreme metamorphism 



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