512 T. Crook — Peculiarities in Phlogopite. 



constricted, but after that it will open out again, like the Vale of 

 Gloucester does in the neighbourhood of Churchdown, Cheltenham, 

 and Tewkesbiuy. 



Further details oi: the development of the rivers in Mid- South 

 Wales must be left for local observers to work, but there is one broad 

 feature to which passing reference may be made. It is the noticeable 

 crescentic sweep of all those important rivers, that, working their way 

 backwards, are effecting such radical changes in the river-distribution 

 of Wales and the neighbouring counties of England. Starting down 

 in the south-west they work their Avay north-eastwards and then 

 gradually curve round north-westwards. 



The Shannon in its crescentic course has nearly traversed Western 

 Ireland. The Teify has almost isolated with the lower reaches of the 

 Ystwyth a somewhat Ireland-shaped mass of Wales. The Towy seems 

 engaged on developing a similar crescentic course, and the great 

 Severn seems bent on making Wales another Ireland. With water 

 stretching from Dee to Severn there would be the replica of the 

 waters that now part Wales from Ireland. 



X. — Optical Peculiakities in Phlogopite. 

 By T. Crook, Assoc. E.G. S., F.G.S. 



IT frequently happens that micas exhibit small basal cleavage rifts 

 of roughly circular shape, and these may be so numerous as to 

 render opaque or feebly translucent a plate which would be otherwise 

 quite transparent. The so-called silver-amber mica of the mica trade, 

 a variety of phlogopite which is highly prized for its low conductivity, 

 apparently owes its silvery sheen to the presence of these cleavage 

 rifts, which overlap each other to a considerable extent, and reflect 

 the light copiously. The presence of these basal cleavage rifts in 

 silvery phlogopite is rendered quite obvious when a plate is examined 

 under the microscope in ordinary light, by the interference rings seen 

 round the borders of the rifts. Such a plate of silvery phlogopite 

 exhibits a peculiar phenomenon in consequence of the presence of 

 these rifts. 



If a plate of suitable thickness, say 0-5 mm. or so, be taken, it 

 will be found to be opaque or only feebly translucent when examined 

 in ordinary light in such a way that the plate is normal to the line 

 of vision. If the plate be now turned about a horizontal line, so 

 that it slopes towards the observer, it will gradually become more 

 transparent, the degree of transparency reaching its maximum at an 

 angle of about 45°, and diminishing as the plate approaches the 

 horizontal position. This greater degree of transparency in an 

 oblique position, in spite of the apparently greater thickness of 

 the plate which the light has to traverse, is a rather puzzling- 

 feature, and one which would scarcely be anticipated. It only occurs 

 in those pronounced silvery types in which the rifts are exceedingly 

 numerous and overlap extensively. The phenomenon seems to be 

 duo to the fact that, in consequence of the overlapping rifts, light 

 can only be transmitted through the plate by a series of oblique 



