i go SCIENCE PROGRESS. 



ciated with the lavas show, on the whole, similar changes. 

 Anthophyllite sometimes takes the place of hornblende. 

 There occur also banded schists, in which alternate layers 

 are rich in green hornblende and colourless augite, respec- 

 tively. The latter mineral is a malacolite variety, and is 

 clearly a metamorphic product. 



The metamorphism of the Devonian strata, consisting 

 of greywackes and conglomerates, also presents some points 

 of interest. In particular, the metamorphosed pebbles still 

 retain their outlines in their metamorphosed matrix. 



These observations, made in the neighbourhood of the 

 large intrusive masses of Saxony, have been cited because 

 they raise several points of general interest. Without 

 mentioning in detail the results obtained in numerous other 

 areas of metamorphism, we may now go on to notice these 

 and some other points in a more general manner. Since 

 the phenomena of metamorphism depend in the first place 

 upon the nature of the rocks metamorphosed, the remarks 

 to be made will be most conveniently grouped in that 

 way. 



Beginning with the argillaceous sediments, which have, 

 perhaps, received more attention than other types of rocks 

 in connection with thermal metamorphism, we may first 

 note as highly characteristic the production in the altered 

 strata of anhydrous aluminous silicates, including under that 

 name the simple silicates of alumina and some more com- 

 plex minerals containing a large proportion of that base. 

 Since these aluminous silicates are, with unimportant excep- 

 tions, absent from all rocks known to have been formed 

 from igneous fusion, they may perhaps afford a useful 

 criterion in some cases. Indeed it would scarcely be too 

 much to say that, when a gneissic or other rock of problem- 

 atical origin is found to contain abundant andalusite or 

 some allied mineral, there is a considerable presumption 

 in favour of regarding the rock as a metamorphosed argil- 

 laceous sediment. In the composition of a normal igneous 

 rock there is a sufficiency of alkalies, lime, and other bases, 

 and of silica, to enable all the alumina to be built up into 

 felspars, pyroxenes, and other common "rock-forming" 



