78 REPOR'T— 1856. 



On the Microscopical Structure of Mica-Schist, By H. C. Sorby, F.G.S. 



The examination of thin transparent sections of mica-schist and the allied rocks 

 shows that there exist two very marked varieties, characterized by the manner in 

 which the flaky crystals of mica are arranged. In one they lie more or less closely 

 in the plane of the alternating layers of different mineral composition^ and, when 

 these are bent into complicated contortions, they also continue to coincide with them; 

 whilst in the other variety they lie in one particular plane, and, instead of varying in 

 direction in sharp contortions, they still remain throughout more or less closely in 

 the same general line. This structure then is similar to cleavage in a contorted 

 slate rock; and its direction in like manner coincides witli tlie axis planes of the 

 contortions, and varies from the general direction in the same particulars. One 

 structure is as if chemical and crystalline changes had occurred in a rock that 

 possessed no slaty cleavage, the arrangement of the particles due to stratification 

 having caused the crystals of mica to be formed in its plane, which may or may not 

 have been subsequently contorted. For this the author proposes the term " stratifica- 

 tion foliation,'' to distinguish it from the other that may be called " cleavage folia- 

 tion," which is as if the rock had been compressed in such a manner as to alter the 

 ultimate structure and develope slaty cleavage, before the large crystals of mica were 

 formed. Then, when the subsequent crystalline changes occurred, the minute flakes 

 of mica, placed more or less closely in the plane of cleavage by the change in the 

 dimensions of the rock, grew up into larger crystals in the same general line of clea- 

 vage. The distribution of these two kinds of mica-schist follows general laws similar 

 to that of cleaved and luicleaved slates.- For instance, in the coast section south of 

 Aberdeen, most of the rocks possess cleavage foliation, whereas in the Loch Lomond 

 district there is simply stratification foliation. 



The author particularly argued that the peculiarities in the rocks having cleavage 

 foliation cannot be explained except by supposing that they have been metamorphosed 

 stratified rocks ; for their structure so clearly shows the effects of both stratification 

 and slaty cleavage, and that the cause- of the separation into layers of different mine- 

 ral composition is pre-existing stratification, and is in no way analogous to that which 

 produced the cleavage of slates — that the cleavage foliation is the effect of previously 

 existing cleavage, and not that slaty cleavage is a partially developed foliation. 



Attention was also drawn to the vast numbers of minute fluid cavities, containing 

 water, that occur in the quartz laj'ers in mica-schist ; being analogous to those found 

 almost invariably in crystals formed from solution, and not in those produced by 

 simple fusion. These indicate that the metamorphic changes have been due to an 

 aqueous process, or else minute globules of water could not thus have been caught 

 in the solid crystals during their formation. Probably an ilevated temperature was 

 also concerned in the change, but not heat alone and a simple partial fusion. 



In mica-schist there is often a peculiar structure, which in many cases might easily 

 be confounded with slaty cleavage. This is when the rock has been so bent into sharp 

 cramples or small contortions, that planes of weakness or actual joint fractures have 

 been produced. These may be so close as to appear just like slaty cleavage to the 

 naked eye, but are seen with the microscope to be quite distinct ; being finite divi- 

 sions, and not an ultimate structure as it is. Both occur occasionally in the same 

 clay-slate, and then give rise to what has been described as double-cleaved slate. 



On some Phenomena in the Malvern District. 

 By the Rev. W. S. Symonds, M.A., F.G.S. 



On the Rocks of Dean Forest. By the Rev. "W. S. Symonds, M.A., F.G.S. 



Researches in Kent's Cavern, Torquay, ivith the original MS. Memoir of its 

 first opening, by the late Rev. J. Mac Enery {long supposed to have been 

 lost), and the Report of the Sub-Committee of the Torquay Natural History 

 Society. By E. Vivian, M.A. 

 A communication was made to this Section by Mr. Vivian, in continuation of that 



which had been given before the Ethnological Section, and extracts were read from 



