196 



Mr. S. Allport and Prof. T. G. Bonney. [June 6, 



(5) Garnet (rare). 



(6) Epidote (?) not common. 



(7) Black opaque minerals : — Iron oxide and perhaps graphite. 

 Tourmaline is extremely rare, three grains occurring in one slide 



only, from close to a junction, and, what is more remarkable, no anda- 

 lusite is distinctly recognisable. One or two other minerals., which 

 may be regarded as " accidental," will be noticed in connexion with 

 the specimens. 



As the effect of contact metamorphism has been described in some 

 detail by one or other of the present authors from the Lake District, 

 Cornwall and Brittany, and by Dr. C. Barrois from the last-named 

 country,* it will suffice to recapitulate very briefly the phenomena, 

 (a) in the case of a shale, (6) in that of a sandstone. 



(a.) Very commonly chiastolite is formed in comparatively large 

 crystals, and this even before the matrix of the rock is very obviously 

 affected. As the junction with the intrusive mass is approached, the 

 chiastolite crystals frequently disappear, and rounded spots of anda- 

 lusite, often very full of mineral enclosures, begin to occur. These 

 after a time become clearer and more crystalliform in outline.f 

 Simultaneously, the colouring matter of the rock collects in specks, 

 rods, and ill-formed crystals, the size of the last increasing on getting 

 near the junction. Mica, usually of a peculiarly rich brown colour, 

 forms in distinct flakes, which are sometimes interrupted by granules of 

 quartz, &c, sometimes locally darkened by black spots, with an inde- 

 finite outline, like an ink blot on porous paper. The basal cleavage 

 is distinct, but the external angles are usually not. well defined. 

 These flakes sometimes exhibit a certain parallelism, and thus impart 

 a foliation to the rock, but this commonly is not well marked. Quartz, 

 generally in clear granules, is also developed ; sometimes it is wholly 

 of secondary origin, but in other cases, probably, original fragments 

 are enlarged, till the rock has become a crystalline aggregate, chiefly 

 consisting of quartz and mica. As a rule, white mica, at any rate 

 in crystals of fair size, only makes its appearance very near to a 

 junction.! 



This brown mica is evidently not a normal biotite. There are in 

 its microscopic aspect certain peculiarities by which it may generally 

 be distinguished ; for instance, the dichroism and absorption are less 

 strongly marked, the colour-change being from pale-yellow to reddish- 



* Allport, 'Quart. Journ. Geol. Soe.,' vol. 32, p. 407 ; Bonney, 'Quart. Journ. 

 G-eol. Soc.,' vol. 44, p. 11 ; Barrois, ' Ann. Soc. Geol. du Nord,' vol. 11, p. 103 ; see 

 also Ward, " Geol. of Lake District" ('Survey Memoirs'), p. 9. 



f On this line of change, as it does not occur in the specimens before us, it is 

 needless to dwell. 



X It is interesting to observe in connexion with this that MM. Fouque and Levy 

 failed to produce artificially, by fusion, white mica, but met with some success in 

 the attempt to obtain biotite (' Synthese des Mineraux et des Roches,' p. 126). 



