82 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



Salisbury Crags. One point of interest is, that although the grains 

 of sand contain many cavities which no doubt, as usual, originally 

 contained water, they have all lost it, as though it* had been ex- 

 pelled by the heat of the igneous rock, in the same manner as it is 

 easily expelled from unaltered quartz by a high artificial tempera- 

 ture. Independent of this, the quartz sand is very little, if at all, 

 chauged. Not so, however, the calcareous shale, which has, to a 

 great extent, been altered into a mass of small, nearly colourless, 

 prismatic crystals having the form and optical characters of tre- 

 molite, or of the pyroxene met with in some slags. This often 

 occurs in small nests along with calcite and quartz, all crystal- 

 lized in situ. The whole rock, however, is of such fine grain, 

 that to the naked eye it appears to be only hardened. Though 

 the general structure is very different from that of hornblende- 

 schists, this rock is of considerable interest in connexion with them, 

 and also as affording good evidence of exposure to a somewhat 

 elevated temperature. 



Development of Mica-schist. 



I have taken much pains in studying the gradual formation of 

 mica-schist from unaltered slates. Apparently the various steps 

 in the change are not always exactly the same ; but, as far as I 

 have been able to learn from the material at my disposal, the order 

 of change was somewhat as illustrated by the following examples : — 



1. A slate close to an igneous rock near Liskeard is composed 

 of thin beds of the usual granular kaolin type, with but little mica, 

 alternating with others of the mica-slate type, containing but little 

 granular matter. Now, in the midst of these materials, evidently 

 deposited as such, there have been formed detached scattered con- 

 cretions, about g-J-^ of an inch in diameter, composed of mica and 

 quartz crystallized in situ, in grains which are on an average some- 

 where about Yw^ ^^ ^^ ^^^ ^^ diameter. These little concretions 

 are, in fact, the very germs of mica-schist, if I may be allowed to 

 use such an expression. An obliquely transverse vein contains 

 larger crystals of mica and quartz, thus clearly proving the transfer 

 of such material from one part to another. 



2. The black chiastolite slate of Ivy Bridge shows some valuable 

 facts. The concretions are larger than those just described, and 

 consist more exclusively of mica. They show only obscure traces 

 of the stratification passing through them ; but it is very distinct 

 in the surrounding material, which has not crystallized in situ. 

 The segregation of mica is well shown, not only by a some- 

 what transverse vein, but also by the alteration of the chiastolite. 

 The altered crystals have a sharp outline, and, with ordinary light, 

 are colourless and transparent from the central cross of impuri- 

 ties up to the extreme edge ; but when examined by polarized light 

 they are seen to be pseudomorphs. The outside, for some depth 

 inwards, has been changed into mica, often with its laminae perpen- 

 dicular to the surface ; and the centre no longer has the strong 



