PRESSURE IN STRATIFIED PAL-SOZOIC ROCKS. 17 



called chiastolite and andalusite is singular. Typical chiastolite 

 generally occurs at a considerable distance from the intruding mass, 

 where the body of the rock is comparatively unaltered. It is excel- 

 lently developed in a matrix which is almost unchanged. Everyone 

 will remember that on approaching the intrusive granite of Skiddaw, 

 as the slaty rock becomes more crystalline by the development of 

 mica and andalusite, the chiastolite disappears. Its regular and well- 

 developed crystals are replaced by vaguely defined spots of granular 

 andalusite, which does not " purge " itself from the associated quartz 

 and mica till we come quite close to the coarsely crystalline granite. 

 This is the case in Erittany (not to mention other places) ; here also 

 the best-developed long prismatic crystals with the peculiar internal 

 structure occur (as at Chapel du Mur and St. Brigette) in a com- 

 paratively not much altered matrix ; indeed at the latter place 

 corals may still be recognized in a rock which contains crystals of 

 chiastolite more than an inch long *. I call special attention to 

 this as indicating the importance of environment ; for, so far as we 

 can tell, there is no essential difference in the materials of the rock 

 itself. 



I have thus traced the development of a rock which may not 

 unfairly be called a mica-schist, though it is a peculiar one. It is 

 composed of quartz and mica, it is truly foliated, it is extremely 

 difficult to identifiy any grain as an original constituent. Eut it is 

 not identical with one of the banded mica-schists from a district 

 where all the rocks are metamorphosed, where we suspect, and 

 perhaps can prove, that they are of Archaean age. The distinction 

 would be recognized at a glance by any one familiar with the two 

 types. Apart from certain mineral peculiarities, not unimportant, 

 well known to those who have made a study of " contact-metamor- 

 phism," there is the marked feature of the general dissemination of 

 mineral enclosures throughout the granules in the quartzose bands. 

 If now we take a banded schist which has not suffered seriously 

 from subsequent pressure (not always very easily found, but still to 

 be obtained by patient searching), we find that the quartz grains, as 

 a rule, are remarkably clear, mica or other minerals only occurring 

 between them in positions where, in a tolerably clean sandstone, 

 there would be flakelets of fragmental mica or earthy matter. If, 

 however, the banded schist has been exposed to considerable 

 crushing, there is sometimes an approach to the structure of the 

 Brittany rock, due to a mechanical admixture of the constituents, 

 as they crush, shear, and slide under great pressure. I have, in my 

 collection, cases of this last from both Anglesey and the Alps ; still 

 even here the distinction between the two kinds of schist can be 

 recognized by a practised eye. 



At the same time, to refer to words of my own, these specimens 

 concerning whose history we have some knowledge, illustrate, though 

 they do not elucidate, those whose history is a matter of conjecture. 

 Once they consisted of stratified sand and mud, and in the bands of 



* I am indebted to my friend the Eer. E. Hill for this information and|for 

 specimens. 



Q. J. G. S. No. 173. c 



