170 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 31, 



sometimes foliated by the chlorite, so as to resemble a green mineral. 

 The annexed diagram, fig. 2, will give an idea of these conditions. 



Fig. 2. — Quartz-vein in clay-slate at Luss, Loch Lomond^ Scotland. 



In general in foliated rocks there is a totally 

 altered structure, and we have many instances 

 where they hardly possess any character in 

 common with cleaved rocks. 



Thus, even the parallelism of the lines cannot 

 be considered invariable ; we certainly have 

 some cases where the foliation coincides vrith 

 the bedding and not with the cleavage, and 

 the property of splitting into laminae in certain 

 directions is often not possessed by foliated 

 rocks at all, particularly if the foliating mineral 

 be not itself of laminar structure. 



We find certainlv that in considerable di- 

 stricts of country the cleaved rocks are totally 

 \ different from those in another district, just as 

 one formation might be expected to differ from 

 another ; but it is doubtful if we ever find 

 such abrupt and total changes as come in such 

 rapid succession in the beds of foliated rocks. 

 (Case 6.) — In a quarry on the roadside about two miles from 

 Crianlorich, in Perthshire, I found the strike N.W.-S.E., with dip 

 30° N.E. ; the rock consisted of very thin beds which alternately pre- 

 sented the character of perfect and highly micaceous mica-schist and 

 of extremely quartzy schist, so that it was not possible to call one 

 single linear foot by the same name. Other cases more unusual will 

 be noticed in the course of the communication. 



The production of foliated structure is not confined to the intro- 

 duction of laminae of one or two mineral substances, as mica, horn- 

 blende, chlorite, &c. ; it may be produced by minerals widely dif- 

 ferent in chemical composition and mineralogical character, and the 

 presence of which is only to be accounted for by the supposition that 

 the constituent elements must have been at hand in the unmetamor- 

 phosed rocks, although in a different state of combination. 



It is apparently also a necessary requisite in the production of 

 foliated arrangement, that the minerals thus formed be of a different 

 chemical constitution to those composing the mass of the rock itself. 

 I may here bring forward some instances of very distinct and de- 

 terminate foliation produced by minerals not usually found under 

 these circumstances. 



(Case 7.) — On the heights immediately above Christiansand, in 

 Norway, probably at a distance of two miles from the town, the 

 section represented by fig. 3 was taken. The prevailing rock is here 

 gneiss (a «), composed of black mica, white quartz, and white and 

 red felspar, with sometimes specks of black oxide of iron. The 

 strike of the foliation runs nearly N. and S., and the dip varies from 



