246 PROCEEDINGS OF THE COTTESWOLD CLUB 



The change from a massive igneous rock to a well- 

 foUated gneiss or schist sometimes produces in an 

 intermediate stage a curious mimicry of a sedimentary 

 grit. The large quarry at the hamlet of White-leafed Oak 

 exhibits this phenomenon. A coarse-grained diorite is 

 intensely crushed. The felspar is broken up into angular 

 fragments, the hornblende being decomposed into chlorite, 

 iron-oxide, and other compounds. As the pressure in- 

 creased, the fragments were rolled out into thin layers, 

 with the soft green chlorite and the dirty-looking iron-oxide 

 lying between the layers, and among the fragments. The 

 resemblance of this rock to a laminated grit is remarkably 

 close. In a further stage of metamorphism, this grit is 

 converted into a well-foliated micaceous schist. 



I must not enter into further details of the new theory 

 of metamorphism. They may be seen in the papers to 

 which I have referred. 



In conclusion, I will indicate the great variety of gneisses 

 and schists which have been formed out of the few 

 varieties of igneous rocks contained in the Malvern range. 



One of the most striking effects of the metamorphism 

 is the production of mica. Three varieties of this well- 

 known mineral have been generated in the Malvern rocks, 

 white mica or muscovite, brown mica or biotite, and a 

 silky white mica called sericite. One or more of these 

 micas will be found in all the gneisses and schists in which 

 the metamorphism has proceeded to an advanced stage ; 

 yet in the granites and diorites which have been the raw 

 material of the metamorphism, not a scrap of mica occurs. 

 All the varieties of mica which are found in the Malvern 

 crystallines must therefore be regarded as of secondary 

 origin. 



These micas are formed in several ways. Potash-felspar, 

 as we have seen, breaks up into quartz and white mica. 

 Soda-lime-felspar may also be a source of mica. But the 



