68 Geological Society : — 



many of the schists of the Malvern Hills were of igneous origin. 

 Thus, mica-gneiss had been formed from granite, hornblen de-gneiss 

 from diorite, mica-schist from felsite, and injection-schists from 

 veined complexes which had been subjected to compression. As a 

 further instalment towards the elucidation of the genesis of the 

 Malvern schists, it was now proposed to discuss the changes which 

 the respective minerals of the massive rocks had undergone in the 

 process of schist-making. 



The schistosity was usually in zones, striking obliquely across 

 the ridge, varying in breadth from a few inches to many yards, and 

 separated from each other by very irregular intervals. Within the 

 zones bands of maximum schistosity alternated with seams in 

 which the original structure had been less completely obliterated. 

 The new structure was connected with a shearing movement, by 

 which the rigid mass was often sliced into countless parallel 

 laminae or flakes. In a more advanced stage of alteration, the planes 

 of movement were obliterated, and a sound clear gneiss or schist 

 was formed. These foliated bands were called " shear-zones." 



The most important shear-zones were those in which diorite was 

 interlaced with granite-veins. The following changes were noticed 

 in tracing the massive rocks into the zones. The hornblende might 

 suffer excessive corrosion, or it might become " reedy " and break 

 up along the cleavages into numerous fragments, which were drawn 

 away from each other in the direction of foliation, or it might pass 

 into chlorite, or chlorite and epidote. The chlorite thus formed 

 often passes into biotite, and sometimes the biotite was changed to 

 white mica. Where shearing was excessive, chlorite sometimes 

 passed directly into white mica. 



Soda-lime felspar was altered to epidote or zoisite, and often to 

 calcite. A more important result was the production of muscovite 

 in the plagioclase. Much of this felspar was reconstructed in small 

 clear crystals or granules. Quartz also was abundantly produced. 

 Diorite might thus be converted either into a gneiss with two micas, 

 or into a gneissoid quartzite. The granite of the veins passed 

 through the usual changes into muscovite-gneiss. 



Other secondary minerals were actinolite (from augite), sphene 

 (from ilmenite), and garnet. 



It was contended that the granite-veins were exogenous, because 

 they appeared as apophyses from large masses ; they had the same 

 coarse texture in different varieties of diorite, and they produced 

 contact-effects similar to those of intrusive veins, including the 

 phenomena of aggregation and enlargement in the minerals of the 

 encasing rock. 



Foreign minerals were often introduced by infiltration. Thus, 

 the hornblende of a diorite was decomposed into chlorite and iron- 

 oxide, which passed for a considerable distance along the shear- 

 planes of an adjacent granite, giving rise to a chlorite-gneiss, and 

 the chlorite was partially changed to biotite. Epidote might be 

 introduced in the same way. 



Both the diorite and the granite of shear-zones tended by loss of 

 bases to become progressively silicified. Most- of the liberated bases 



