CRYSTALLINE SCHISTS OP THE MALVERN HILLS. 



527 



Felsite. 



Near the Wind's Point, there is a well-marked band of felsite 

 hading in accordance with the banding of the enclosing gneiss. 

 It is compact, homogeneous, and of a pale-reddish colour. A 

 similar rock occurs in the Raggedstone Hill, and by its modifica- 

 tion gives rise to schists. 



2. Evidence of Pressure. 



Zones of crushing, indicated by bands of breccia, are very common 

 in the district ; but some of these are posterior to the metamorphism 

 and do not concern us. Some direct evidence of mechanical force, 

 resulting in schistosity, will come out in describing the intimate 

 structure of certain rocks ; but it is obviously difficult to obtain 

 very abundant indications amongst igneous masses, where there are 

 no beds to be contorted and faulted. There is reason to believe 

 that the pressure is partly transformed into molecular energy, and 

 thus changes are brought about which often mask mechanical effects. 

 There is, however, some field-evidence for the action of enormous 

 pressures. A good example is seen in a quarry of gneiss at the 

 southern termination of Key's End Hill. The foliation-dip is 

 at a moderate angle to the south-east. The rock is traversed by 

 contorted veins of granite, running in several directions. One of 

 these is shown in fig. 1, and gives a rough measure of the pressure. 



Pig. 1. — Contorted Granite-vein in Gneiss. 

 S.W. N.E. 



Assuming the vein originally to have been straight, its length has 

 been reduced in the proportion of about five to two. 



The regional pressure has acted with intensity only at intervals 

 along the range. In the North Hill and the northern part of the 

 Worcestershire Beacon there are large masses of diorite which have 

 undergone little mechanical change ; but between the summit of 



eBeacon and the Wych there are numerous alternations of diorite 

 and granite displaying foliation.' The long ridge between the 

 Wych and the Wind's Point contains a great deal of black diorite 



2n2 



