RESEARCHES IN THERMAL METAMORPHISM. 301 



some relation between the particular constituents added to 

 the metamorphosed rocks and the nature of the magma that 

 furnished such constituents, and here the facts seem very 

 difficult of explanation. The observed enrichment in silica 

 and alkali is found not near intrusive rocks rich in those 

 constituents, but especially in contact with diabases. 



Brogger, as a result of his earlier researches in the 

 Christiania district, laid down the rule that the phenomena 

 of metamorphism are of the same kind, whatever the nature 

 of the intrusive rock that has produced them. Recently he 

 has found cause to modify this conclusion (31). At Solvs- 

 berget, in the neighbourhood of Gran, the argillaceous shales 

 with Ogygia are metamorphosed by a considerable intrusion 

 of a basic rock described as olivine-gabbro-diabase. They 

 are altered first into dark violet " hornstones " with increas- 

 ing development of minute flakes of mica, and then into 

 a visibly crystalline rock enclosing crystals of plagioclase- 

 felspar more than 5 cm. in length. Near its contact with 

 the basic intrusion this highly altered rock contains a con- 

 siderable amount of hypersthene, and the magnesia and 

 ferrous oxide of this mineral are supposed to have been ob- 

 tained from the intruded magma. The same shales where 

 metamorphosed by a more acid rock — the nordmarkite or 

 quartz-syenite of the district — do not contain the exceptional 

 mineral in question. The author writes guardedly, and the 

 evidence is clearly incomplete without bulk analyses of the 

 altered and unaltered shales. 



Another question of considerable interest is that of the 

 interchange of material within the mass of a rock under- 

 going thermal metamorphism, apart from any accession of 

 matter from without (32). The very variable characters of 

 many metamorphosed rocks and the preservation of such 

 structures as the spherulitic, the amygdaloidal, and the 

 laminated seem to indicate that any redistribution of 

 material in the rocks during metamorphism is confined to 

 very narrow limits. In other words the new minerals pro- 

 duced at any point seem to depend on the original chemical 

 composition of the rock in the immediate neighbourhood of 

 that point. In this connection the metamorphosed amyg- 



