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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 

 on which they are laid down, and tracing in detail the vertical 

 planes which bound the arches. Commencing on the south, the 

 first vertical plane runs about four miles within the Highland border, 

 with a mean direction of about N. 55*^ E. : it crosses more than 

 once the junction of the clay slate and mica schist. South of this 

 plane the cleavage of the slate forms the beginning of an arch, 

 which ends abruptly at the junction of the slate with the Old Red 

 Sandstone. 



To the north of this vertical plane four arches run across the High- 

 lands : the most southern of these, with a diameter of ten or twelve 

 miles, is formed partly of the cleavage of the slate, and partly of the 

 foliation of the mica schist. The hills on the south side of Loch 

 Tay coincide with its central axis. The vertical plane which forms 

 its northern boundary crosses Ben Lawers, and has a mean direction 



