Vol. 60.] METAMORPHIS.U IN THE LOCH-LOMOND DISTRICT. 



13 



JC - 



S 



the crushed material, and the resulting rock may be much more 

 highly-crystalline than a rock of similar composition which has 

 escaped the shearing and crushing to some extent. 



As the present state of any mass of rock de- 

 pends on the extent to which each of these pro- 

 cesses took effect, it is evident that the type 

 and nature of the folding become very 

 important factors in regional metamor- 

 phism. With these preliminary observations, 

 which sum up the conclusions arrived at by 

 other workers among metamorphic rocks, as well 

 as my own observations during seven years of 

 study of folded rocks, I may now proceed to a 

 description of the folding in the area in question. 



IV. Nature op the Folding, and Stratigraphical 

 Relations. (Fig. 1.) 



The folding of the Leny Grits and Aberfoil 

 Slates, the groups lying nearest to the Highland 

 Border, is not very well seen, as a well-marked 

 cleavage has been produced in all the finer beds, 

 and has even affected the coarser grits to some 

 extent, with the result that the bedding is often 

 obscured. Where actual dips can be observed, 

 however, they are nearly always at high angles, 

 while the mapping out in detail of certain 

 grit-bands shows that the folding, though never 

 very deep, is somewhat tightly ' packed.' The 

 magnificent sections afforded by the slate- 

 quarries at Aberfoil and Luss confirm this obser- 

 vation. The cleavage is usually approximately 

 vertical, and often coincides with the bedding. 



Passing north-westward and ascending in the 

 series, the cleavage gradually disappears, the finer 

 beds showing traces of it long after the more 

 massive beds have ceased to give any clear evi- 

 dence of such a deformation. A shear-structure 

 becomes more noticeable as the cleavage declines, 

 and the folding is easily recognizable. The rocks 

 belong to the Beinu-Ledi Group, and are chiefly 

 fine quartzose grits, becoming coarser in the 

 higher members. A compound syncline and 

 anticline are disclosed, formed of folds which 

 are practically isoclinal, with nearly vertical axial 

 planes. This flexuring of the massive Beinn- 

 Ledi Grits is a very important factor in the 

 metamorphism of the district, as the coarse upper 

 beds thrown into great vertical folds in the anticline seem to have 

 formed a relatively rigid barrier, against which the strata on either 



O J= 



ih 



