388 Royal Society. 



but also capable of producing a sensible change in the place of the 

 axis of rotation. 



Feb. 19. — The reading of Mr. Sharpe's paper, " On the Arrange- 

 ment of the Foliation and Cleavage of the Rocks of the North of 

 Scotland," commenced at the last meeting, was resumed and con- 

 cluded. 



The author applies the term, cleavage or lamination, to the divi- 

 sional planes by which stratified rocks are split into parallel sheets, 

 independently of the stratification ; foliation, to the division of crystal- 

 line rocks into layers of different mineral substances ; slate, to strati- 

 fied rocks intersected by cleavage ; and schist, to foliated rocks only 

 which exhibit no bedding independent of the foliation. 



He considers that no distinct line can be drawn between gneiss 

 and mica schist, chlorite schist, &c, which pass from one into the 

 other by insensible gradations; have the same geological relations, 

 and foliation subject to the same laws. He states that their boundaries 

 have been laid down arbitrarily on the published maps of Scotland. 

 The quartz rock of Macculloch includes two formations ; the one, 

 a quartzose variety of gneiss, included in this paper under that head; 

 the other, a stratified sandstone altered by plutonic action. 



The author treats the foliation of gneiss and schist as a series of 

 simple curves, obtained by observing the general direction, and dis- 

 regarding the minor and more complicated folds. The convolutions 

 are usually greatest where the dip is slightest, but where the folia- 

 tion is vertical or nearly so, it usually follows true planes without 

 contortion ; thus the most correct observations are those taken where 

 the foliation is vertical. 



When the foliation of gneiss and schist is traced over extensive 

 areas, and the minor convolutions disregarded, it is usually found 

 to form arches of great length and many miles in diameter, bounded 

 by vertical planes, between which the inclination increases with the 

 distance from the axis. Each arch is succeeded by a narrow space 

 in which the dip is irregular, and beyond which another arch com- 

 mences of a form similar to the first. Portions of two adjoining 

 arches seen without the rest form the fan-like structure observed by 

 several geologists. The arrangement of the foliation in arches cor- 

 responds with that of the cleavage of the true slates previously de- 

 scribed by the author, except in the greater convolution of the gneiss 

 and schist. 



Along the southern border of the Highlands a band of stratified 

 clay slate rests on mica schist: at the junction, the foliation of the 

 schist conforms to the cleavage of the slate, and the two together 

 form an arch, but there is no connection between the stratification 

 of the slate and the foliation ; moreover, the divisional planes cross 

 from one rock to the other, without change of direction, being planes 

 of foliation in the mica schist, and of cleavage in the slate : these 

 facts confirm Mr. Darwin's opinion, that cleavage and foliation are 

 due to the same cause. 



The author describes the parallel arches of foliation which cross 

 the Highlands, illustrating his description by sections and a map 



