RESEARCHES IN THERMAL METAMORPHISM. 295 



metamorphosed by the Carrock Fell gabbro, the bastite 

 pseudomorphs after hypersthene being converted into horn- 

 blende (20). 



The basic and intermediate tuffs near the Shap granite 

 behave in general very like the associated lavas, though 

 with some differences. Brown biotite is always the most 

 conspicuous mineral in the metamorphosed rocks, and gives 

 them the character of mica-schists or fine micaceous gneisses. 

 A marked foliation is usually evident, the mica flakes being- 

 developed parallel either to the bedding or the cleavage of 

 the tuffs, and a similar foliation often follows the flow lines 

 of the lavas. The metamorphism of some Carboniferous 

 volcanic tuffs on Dartmoor has been described by McMahon, 

 the chief point of interest being the production of antho- 

 phyllite, a mineral not yet recognised in the Shap district. 

 It occurs in radiating bundles of needle-like crystals with 

 the usual characters (22). 



But little detailed information has been collected relative 

 to thermal metamorphism in the various types of crystalline 

 schists and allied rocks. Among contributions to this sub- 

 ject in recent years the late G. H. Williams' account of the 

 metamorphosed rocks on the Hudson River (23) is of 

 special interest. It illustrates the changes set up in a series 

 of mica-schists invaded by massive intrusions of diorite. 

 Approaching the intrusive rock, the mica-schists are found 

 to become more and more contorted and filled with " eyes " 

 or lenticles of quartz containing garnet and other "contact 

 minerals " — a feature noticeable in the metamorphosed 

 flags of the New Galloway district already mentioned. 

 In the mica-schists themselves have been developed stauro- 

 lite, sillimanite, cyanite, and garnet, the completeness of the 

 metamorphism increasing in the direction of the contact, 

 where the schistose character is wholly lost and the rocks 

 become hard and massive. In the progressive stages of 

 metamorphism observed a gradual disappearance of the 

 quartz and muscovite of the schists is found to accompany 

 the development of biotite, sillimanite, staurolite, and garnet. 

 This points to a chemical as well as a mineralogical change 

 in the composition of the rocks, and indeed analyses show 



