317 



bright red by alteration. In the deformed rock-masses the original 

 micro-structure of a granite has been more or less replaced by that of 

 a gneiss or schist. A parallel structure has been developed. This 

 may or may not have been accompanied by the development of white 

 mica. When at all pronounced it is invariably accompanied by the 

 development of micro-crystalline or granulitic aggregates of quartz and 

 felspar and these aggregates by their distribution in the rock, and 

 especially by their relation to the larger grains of quartz and felspar, 

 serve to define the parallel structure. Sometimes the aggregates are 

 so fine as to be incapable of resolution into distinct grains in which 

 case they must be described as crypto-crystalline. They occur as 

 lenticular stripes and bands of greater or less width which wind in 

 and out amongst the larger constituents and produce what may be 

 aptly termed a micro-flaser structure. A good example of a gneiss 

 ol this type occurs above Kynance at the point where the road 

 begins to descend. It forms a band in the serpentine and is exposed 

 in the floor of the road. Under the microscope it is seen to consist 

 of large irregular felspar-grains which are partly striated and partly 

 not. These grains give undulose extinction and are sometimes broken ; 

 the parts not having been far removed from each other. They are 

 arranged with their longest diameters parallel with the foliation and 

 are sometimes traversed by cracks at right angles to the foliation. 

 The ground-mass is a micro- or crypto-crystalline aggregate mainly 

 composed, it may be presumed, of quartz and felspar. Owing to a 

 variation in the size of the grains of different portions of this 

 aggregate and to the existence of a certain amount of uniformity in the 

 orientation of the grains in certain patches a marked parallel structure is 

 apparent under crossed nicols. Narrow lenticular stripes and patches 

 are recognizable and these sweep round the larger constituents in the 

 graceful curves suggestive of fluxion structure. There is a little biotite 

 recognizable and there are dark wavy lines without definite character 

 which may represent the mechanically disintegrated remnants of 

 this mineral. 



A light-coloured gneissose rock occurs above the shore near 

 Porthalla. It has been described by Prof. BONNEY.W This rock is more 

 or less fissile, the planes of schistosity being coated with a white mica. 

 In micro-structure it bears a close relation to the rock just described 

 except that white mica has been extensively developed along certain 

 planes. This mica occurs partly in the condition of definite but ragged 

 crystalline scales, and partly in a fine-grained aggregate. Colonel MACMAHON 

 has proposed that mica in the latter condition should be termed crypto- 

 crystalline. The larger crystalline constituents in this rock are similar 

 to those of the rock from Kynance. They lie in a micro-crystalline 



(1) Q.J.G.S., Vol. XXXIX., p. U. 



(2) Microscopic structure of some Dalhousie rocks. Records of the Geol. Survey of India. 

 Vol. XVI., JPt. 3 (1883), p. 131. 



