V. 



ROCK-STRUCTURE. 



surfaces are so related to one another as to produce octahedral 

 and prismatic shapes, which emulate regular crystals. Some of 

 these are represented below ; and I may say in general that they 

 fairly approximate to secondary forms of anorthic felspar, common 

 hornblende, and augite. It is conceivable, and in fact appears on 

 examination to be true, that in the seemingly confused crystalliza- 

 tion of the masses, planes of easy divisibility may have been 

 sometimes occasioned by the greater than ordinary prevalence of 

 particular cleavage-parallels through small masses of rock. They 

 occur in quartzo-felspathic, dioritic, and aphanitic compounds, and 

 the forms are different in each case. 



Diagram XVI. 



1. Crystalloid mass of large-grained quartzo-felspathic granite, with very little 

 mica, and veins of epidote. The felspar is red, and well crystallized ; the quartz 

 grey. The form of this specimen appears to be on a type of albite. 



2. Crystalloid mass of large-grained dark hornblende and reddish felspar ; the 

 former well crystallized, the latter but slightly so. This felspar is anorthic. One 

 of the octahedral faces is of recent fracture, the others are natural faces. The form 

 is derivable from a prism of hornblende with lateral solid angles replaced. 



3. Finer-grained mixture of the same minerals. 



Thus we have in the Malvern ridge of very old rocks : 

 Gneiss, varying from a distinct crystallized mixture of felspar, 

 quartz, mica, and hornblende (North Hill), to an obscure, grey, 

 much-fissured mass, in which the component parts are hardly 

 distinguishable (about the Wych). In respect of lamination 

 very unequal ; often no sign of it. An intermediate condition 



