166 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 31, 



2. On the Causes producing Foliation in Rocks ; and on some 

 observed cases of Foliated Structure in Norway and Scot- 

 land. By David Forbes, Esq., F.G.S., A.I.C.E. 



The study of the metamorphic and crystalline rocks has of late 

 years attracted much attention, and the researches of Darwin, Sharpe, 

 and others have brought forward many facts particularly interesting 

 and important. 



It will probably, however, be admitted, that, notwithstanding 

 the labours of these geologists, great diiference of opinion exists 

 as to the formation and structure of these rocks, and that at present 

 we are not in possession of sufficient data to enable us to place the 

 question upon a firm basis. It is therefore of importance that as many 

 observations as possible be laid before geologists working at this 

 subject ; and it is consequently hoped that the present communication 

 may not prove unserviceable in advancing the inquiry. 



The observations here brought forward have with a few exceptions 

 been made during a residence of some years in Norway, where the 

 foliated rocks play so important a part. The others are the result 

 of some short excursions in Scotland made for the express purpose 

 of comparison. 



For a long period, and it may be said until late years, it was the 

 generally received opinion that the lines of foliation in rocks were 

 lines corresponding in direction or identical with the lines of stratifi- 

 cation, and were produced by the alteration of these lines by meta- 

 morphic action. This view of the case can no longer be considered 

 tenable, and the observations of many geologists have shown that 

 foliation frequently takes place at a different angle to that of strati- 

 fication. 



Darwin seems to consider this as almost invariably the case, and 

 remarks that, even should they correspond in the strike, they would 

 not correspond in the dip ; and Ramsay, in his paper " on the Lower 

 Palaeozoics of North Wales," considers that there is no doubt that in 

 many cases the foliation of the Anglesea rocks runs much across the 

 dip. Many other observers might be quoted on this point ; but, on 

 the other hand, several geologists of high reputation seem to con- 

 sider these cases as exceptional, and that in general the foliation 

 is developed according to the lines of stratification. 



(Case 1.) — As bearing on this point, I observed in the Highlands 

 of Scotland, near Crianlorich, on the road to Tindrum, in Perthshire, 

 regular beds of a blue limestone, resting upon a dark greenish-grey 

 mica-schist, and dipping 32° N.E., with an inclination in the course of 

 the strata of 24° S.E. This limestone was most completely foliated 

 by the introduction of small plates of white mica, so that it often 

 could hardly be distinguished by the eye from gneiss. Here the 

 foliation in the limestone appeared perfectly identical with the planes 

 of bedding, as shown by the coloured bands and large beds of the 

 limestone. This was apparently also the case in the gneiss ; but in 

 the crust or stratum above the limestone on the surface of the ground 

 the foliation was very twisted and irregular. 



