101 . MB. E. H. WOETH OX THE [vol. IXXV, 



and that the ground-mass consists of a mosaic of quartz and felspar, 

 • which some writers seem to regard as proof of dynamo-meta- 

 morphism." . He. sees no reason why. in this case, these peculiarities 

 are not sufficiently accounted for by supposing that they were 

 produced when the granite was forced through the jaws of a fissure 

 in the slates, or by strains in the dyke when solidifying. 



The ' mosaic of quartz and felspar ' is, in my slides, no other 

 than typical granitic structure in miniature. Our normal Dartmoor 

 granites rarely show the broken felspars, partly on account of the 

 absence from them of those small, elongated, rectangular sections 

 which are frequent in the Melclon aplite. Such sections are 

 peculiarly liable to injury by pressure or movement. In the 

 present instance the breakages probably occurred when the dyke 

 was solidifying. It appears to have been injected in a singularly 

 fluid condition, the evidences for which are the distance to which 

 veins only a fraction of an inch thick have penetrated the shales ; 

 the extremely fine grain of some parts of the dyke which have 

 been chilled by contact with the shales, and the practical absence 

 from such parts of any but very small felspar-laths (PL YII, 

 fig. 13 i ; and, on the other hand, the occasional arrangement of 

 minerals at the surface of some inclusions. The inclusions where 

 small may be assumed to have been thoroughly heated, and to 

 have exercised no real chilling action ; the nearest felspars not 

 infrequently lie with their lengths normal to the junction-plane 

 (PI. YII, fig. 1-1), very much like the minerals on the walls of come 

 metalliferous veins. It would appear very doubtful whether, when 

 the aplite was forced through the jaws of the fissure in the shales, 

 there were any laths of felspar already developed. 



This breakage of felspars is neither universal nor general 

 throughout the mass: most crystals in most sections are uninjured, 

 some sections show more breakages than others, not a few show 

 none. That there should have been strains followed, not only from 

 the necessary pressure-reaction of the walls of the dyke, but also 

 from the pressure of the shale-inclusions. Notwithstanding the 

 fluidity of the rock when it was intruded, it must have presented 

 some viscosity, or the inclusions could not have floated. Part at 

 least of the included rock has a specific gravity of 2*90, whereas 

 the specific gravity of the aplite is but 266 : hence flotation 

 unaided by viscosity was impossible, and the weight of the inclu- 

 sions must have strained their surroundings : it is in their vicinity 

 that broken and bent felspars are most frequent. 



Felspar. — Both orthoclase and plagioclase are present, the 

 latter being dominant. In the normal rock the orthoclase is 

 anhedral. rather rarely presenting crystal faces. An ordinary 

 diameter for its grains might be set at 0*5 mm., but some would 

 be much smaller, and others would reach 1 mm. and over. Small 

 crystals of albite are not infrequently present as inclusions, with 

 sometimes a needle of tourmaline or a blade of mica, both the 

 latter being very minute. In the coarser veins (pegmatite of some 

 writers) all the felspars have a tendency to fair crystal outline. 



