Dr. Irving — The Malvern CryBtallines. 461 



developed accessory minerals, such as kyanite, staurolite, and garnet. 

 It seems doubtful if anything more than ''cleavage-foliation" is to be 

 seen in these cases. 



(iv.) Shear-planes. — These are very conspicuous, structural features 

 of the rock-masses exposed to view in the extensive quarries at 

 North Malvern, at West Malvern, and at the Hollybush Pass. They 

 seem to be the " divisional planes " of Mr. Rutley. Occasionally, as 

 in the largest quarry at North Malvern, a rough parallelism may be 

 observed in their arrangement, but more extensive observation shows 

 that this is only a local and accidental circumstance. Generally they 

 cut through the rock-mass in all directions and at all angles to the 

 quarry-face, and the evidence they afford of sliding movements justifies 

 one in regarding them as entirely mechanical in their origin, and due 

 to crushing on a grand scale, acting along planes of weakness in the 

 rock, quite similar to the crush -faulting which has been described 

 by Dr. Stapff in the St. Gothard granite cut through in the railway 

 tunnel.^ They frequently intersect at all angles what we may call 

 the " master-joints " of the rock, which appear to owe their origin to 

 the shrinkage of the mass as a whole during solidification ; and they 

 are not in any way petrographically related to the adjoining holo- 

 crystalline, and often massively-crystalline rock. It seems impossible, 

 therefore, to regard them as traces of anything of the nature of bedding. 

 The mineral changes wrought by dynamic action on these divisional 

 planes in the moimtain-building stage, appear to be entirely in the 

 direction of degradation of the basic constituents of the adjoining 

 rock into oxides of the heavier metals (chiefly haematite), after pul- 

 verization of the rock for the thickness of a few millimetres. They 

 are often conspicuously slickensided. Possibly some of the iron 

 oxide may be present as carbonate, synthesized by the action of 

 carbonated vraters from the surface in the presence of organic matter 

 in solution in such waters reducing the peroxides to protoxides ; 

 but these results are altogether secondary, and have nothing to do 

 with the genesis of the rocks. The same remark probably applies 

 to epidote, which is rather abundant as an infilling material in the 

 smaller cracks of the diorite (especially at North Malvern), and vs^as 

 derived in all probability by the agency of infiltrating water from 

 the hornblende. 



(v.) Crush-planes and zones. — Where the original rock was very 

 hornblendic this has been degraded into what some vs^ould call a 

 " chlori tic- schist," and some of these have been at times described 

 by eminent writers under this term. Yet they can hardly be 

 correctly termed schists at all ; for they are so incoherent that it is 

 impossible to get a piece capable of being sliced. They might, 

 perhaps, be described as rotten slate w^ith a very irregular cleavage. 

 The original hornblende appears to have been completely pulverized 

 in the presence of interstitial water, and, by the help of this water 

 (or by water introduced subsequently), to have been wrought into a 

 fine pasty mass, upon which a crude cleavage-structure has been 

 induced by pressure. Several narrow zones of such chloritic masses 

 1 See Geological Magazine, 1892, p. 6. 



