ANNIYEESAET ADDRESS OF THE PRESIDENT. 



97 



plicated manner ; sometimes strain-slip cleavage, or, as a variety of 

 this, overfold strain-cleavage, is produced. The constituent minerals 

 exhibit to a greater or less degree indications of fracture and dis- 

 turbance, obviously very much depending on the form and nature 

 of the mineral ; but, perhaps owing to the absence of felspar, there 

 is not quite so much of crushing and of mineral change as in the rocks 

 described above. Still, mineral deposit takes place along the planes 

 of cleavage, sheen-surfaces are formed, and these ma}^ be traced in 

 every degree from the stage where they are quite subordinate to 

 the stratification-foliation, to that in which the latter has been 

 wholly obliterated by cleavage-foliation, when the rock splits up mto 

 thin slabs, sometimes almost films, of a friable micaceous schist, 

 which we might almost call a slate (figs. 1 & 2). 



Fig. 2. 



Fig. 1.— Fragment of Lepontine Gneiss (about S^ inches long) showing stratifi- 

 cation-foHation — darker bands (a) chiefly biotite ; lighter, chiefly quartz, 

 with some felspar ; — the oleavage-foHation, indicated by filmy white mica, 

 parallel with line (b) and perpendicular to the paper. The flakes of dark 

 mica exhibit a certain parallelism to the plane through (b). IS^ear Altanca, 

 north side of Val Bedretto. 



Fig. 2. — Fragment of Calc-mica Schist (about 3 inches long) showing stratifica- 

 tion-foliation afiected by cleavage-foliation. («) Bands of greenish mica, 

 (b) the direction of the clearage-foliation, indicated on the faces of the 

 specimen (not seen) by platy surfaces, spotted with filmy white mica ; the 

 flakes of mica in (a) tend to be parallel with the plane through (b). From 

 above the Tremola schists, north side of Lago di Eitom, Yal Piora. 



Yery interesting changes of a like nature will be found on the 

 Aberdeenshire coast, north of Stonehaven. At Muchalls, the rock 

 which forms the crags is commonly a very fine-grained quartzose 

 gneiss, the mica layers (chiefly biotite) being from -1 to '2 of an 

 inch apart. These are greatly zigzagged and corrugated ; in some 

 places the surface of a block is seen to be traversed by bent and 



