8. AlZport — Rocks of Brazil Wood, Charmoood Forest. 483 



remarkable that hornblende should be absent, as it occurs in all the 

 other granites of the district. 



The junction with the schist is interesting, as it presents two 

 varieties. In one specimen the junction is as sharp and distinct a 

 line, as in any of the Cornish examples described in the paper 

 already quoted ; in others, it is not so well defined ; the two rocks 

 have, in fact, been so completely blended together, that there is a 

 narrow band of such indefinite character, that one can hardly say 

 to a quarter of an inch where one begins and the other ends. A 

 thin slice shows under the microscope a narrow belt between normal 

 granite and normal schist, which consists almost exclusively of 

 imperfectly developed felspar crystals and mica. This felspathic 

 band is also marked by the presence of numerous minute black 

 grains of magnetite, which are here and there loosely aggregated in 

 clusters. The slice contains small garnets in both the granite and 

 schist. 



Micaceous Schist. — The rock into which the granite has been 

 intruded has been called a peculiar or unique gneiss ; it is, in fact, 

 so peculiar that the term is inappropriate, as the rock has neither 

 the mineral composition nor the structure of gneiss; it contains no 

 recognizable felspar, and there is no true foliation. It is tough and 

 difficult to break, though rather more fissile in one direction than in 

 others ; it is purplish grey in colour, and glitters with numerous 

 bright flakes of mica. The description of its microscopic structure 

 given by Professor Bonney in the paper already cited agrees in the 

 main with my own observations ; in my slices there is, however, 

 so little of the " fine granular base," that I should prefer to describe 

 it as a rock consisting almost entirely of two micas, with a few 

 grains of quartz, and numerous minute black grains (doubtless 

 magnetite) irregularly scattered through the mass ; and, in addition, 

 there are occasional small patches of the fine granular substance 

 showing aggregate polarization ; it occupies spaces between the 

 flakes of mica, but exhibits no crystalline forms. There are also 

 small flakes and streaks of clear red ferric oxide. The brown and 

 greenish-brown mica has all the characters of altered biotite. The 

 white mica is more abundant, and occurs here and there in rather 

 large flakes ; in its brilliant chromatic polarization, and other 

 optical characters, it agrees perfectly with a white mica in the 

 altered clay-slate in contact with the granite of St. Michael's Mount, 

 Cornwall. As it is not quite like muscovite, it may probably be 

 paragonite as suggested to me by Prof. Bonney ; and this would 

 agree well with the analysis given by him at p. 224, as that mineral 

 contains from 36 to 40 per cent, of alumina. As regards structure, 

 there is no approach to foliation in any of my slices ; the con- 

 stituents lie in all possible directions without any definite arrange- 

 ment. There is, however, a banded appearance in some specimens 

 caused by the crystallization of the two micas in rather larger plates 

 along roughly parallel lines. Near the junction with the granite 

 the schist contains many small garnets, which are interesting micro- 

 scopic objects. 



