410 S. ALLPORT ON THE METAMORPHIC ROCKS 



nodules are elongated in the direction of the irregular planes of 

 foliation, — a clear indication, I think, that in this instance meta- 

 morphism was effected under an amount of pressure sufficient to 

 interfere with the process of crystallization ; or possibly the nodules 

 were originally spherical, and were forced into their present form 

 while the mass was in a somewhat plastic condition. 



A singularly interesting example of metamorphism is afforded by 

 a specimen of altered slate from Botallack, collected close to the 

 granite, and represented in PL XXIII. fig. 6. On examining a 

 thin slice, the first thing that strikes one is the occurrence of innu- 

 merable minute black grains of magnetite arranged in rather wavy 

 lines of foliation, which traverse the section from side to side, and 

 occasionally bend round small patches of quartz. Then there are 

 numerous long flat blades of nearly colourless tremolite, many of 

 which are arranged in radiating groups, while others lie at all 

 angles to each other ; and thickly scattered through the quartzose 

 matrix there are crowds of small flakes of lepidolite, none of which 

 are to be found in the tremolite. The most remarkable and inter- 

 esting point is, that the undulating parallel lines of black granules 

 pass uninterruptedly from the base through and across the blades of 

 tremolite, several of which are thus traversed by two or three rows 

 of granules ; and as the flakes of mica also contain them in consi- 

 derable numbers, it is evident that the black grains must have been 

 arranged in lines prior to the crystallization of either tremolite or mica. 

 We have here, I think, the distinct record of two great changes in 

 the structure of the rock — first its conversion into foliated schist, 

 and subsequently the development in it of the tremolite. But, 

 during the process of crystallization of this mineral, the structural 

 character of the mass became greatly changed ; and the final result 

 has been the production of a highly crystalline rock in which the 

 bands of black granules constitute the only vestige of its previous 

 foliated texture. 



There is no doubt as to the hornblendic character of the bladed 

 prisms ; for, by cutting slices at right angles to each other, there are 

 seen several excellent transverse sections of the four-sided prism with 

 opposite angles of 124° 3'. 



Cape Cornwall. — Specimens containing junctions of granite and 

 slate from Cape Cornwall present the same general characters as 

 those from Mousehole, but also occasionally afford some interesting 

 varieties of texture. In one thin slice the granite contains many 

 small crystals of tourmaline irregularly scattered through it, while 

 parallel with the line of junction, yet still well within the granite, 

 they form a narrow band composed of numerous prisms, and associ- 

 ated with them are many plates of white mica ; then there is a well- 

 defined sharp junction with a fine-grained micaceous schist con- 

 sisting of green, white, and red mica, together with quartz and 

 tourmaline. Close to a large granite-vein seen on the shore, there 

 is an imperfectly foliated schist of very similar character to the one 

 previously described from Botallack : it contains similar lines of 

 black granules and red mica ; but instead of tremolite, there is a clear 



