PROCEEDINGS OF THE COTTESWOLD CLUB 247 



most remarkable change that takes place in the Malvern 

 mctamorphism is the generation of mica out of hornblende. 

 The hornblende yields, as a product of decomposition, the 

 soft green mineral called chlorite, a siHcate of magnesia 

 combined chemically with water. In a further stage of 

 mctamorphism, this chlorite loses water and takes up 

 potash, thus becoming a brown mica. It would seem 

 then that all the principal minerals in the Malvern igneous 

 rocks, except, of course, quartz, may be a source of mica. 



Granite alone is converted into muscovite-gneiss, as we 

 have already seen. Diorite, a compound of hornblende 

 and soda-lime-felspar, is changed to a hornblende-gneiss, 

 and in a further stage into biotite-gneiss. Sericite-gneiss 

 also may be formed out of diorite. Some of the most 

 interesting gneisses are produced by the intervening of 

 diorite and granite. The contact of the granite with the 

 diorite assists in the production of brown mica, and when 

 the veins are numerous and near together, the intervening 

 diorite becomes charged with the mica, and a beautiful 

 gneiss results, in which the bands of red granite alternate 

 with dark seams glittering with the mica. 



These illustrations will perhaps suffice to explain the 

 general theory of the mctamorphism of the Malvern 

 crystalhnes. A similar theory has been found to apply 

 to the old gneissic rocks of the Highlands of Scotland, of 

 Scandinavia, and of many other parts of Continental 

 Europe, as well as to the so-called Laurentian gneisses of 

 North America. Indeed, it is now generally admitted that 

 all the older Archaean rocks of the globe are of igneous 

 origin. 



Q2 



