86 Mr Sharpe on the Foliation and Cleavage of 



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 Highlands ; 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 of N. 50° E. 

 The next arch northward, consisting principally of gneiss, has 

 a diameter varying from twenty-five to thirty miles ; its axis 

 runs for some distance along the central ridge of the Gram- 

 pians. The granite of Cruachan and Ben-Muich-Dhui inter- 

 feres with the regularity of the foliation of this district, and 

 the lines are thrown to the north by the granite of Aberdeen- 

 shire ; the line which bounds this arch on the north crosses 

 the Spey near Laggan, and runs N. 40° E. through Corbine 

 into the Monagh Leagh mountains. To the north of that 

 line, the foliation of the gneiss forms an arch only ten miles 

 wide, bounded on the north by a vertical plane running N. 

 35° E., which crosses Coryaraiek. This plane forms the south- 

 ern boundary of an arch, varying from fifteen to twenty-five 

 miles wide, entirely of gneiss, bounded on the north by a band 

 of vertical foliation which runs about N. 30° E., from Glen 

 Finnan through the middle of Ross-shire, and across Ben 

 Nevis. To the north-west of this band there is half an arch 

 in the foliation, varying from twenty to thirty miles wide, 

 which ends abruptly at a line to be drawn from Loch Eribol 

 and Loch Maree, on the west of which the gneiss is uncon- 

 formable to that hitherto described, but agrees with that of 

 the island of the Lewis, forming a series of arches which run 

 about NW. 



From the want of parallelism in the lines of foliation of the 

 Highlands, they would all nearly converge between Lough 

 Foyle and Lough S willy among the mica schists of the north 

 of Ireland. 



The most rugged and elevated hills are usually on or near 

 the lines of vertical foliation ; the axis of the arches are ge- 

 nerally found in high land, and the principal valleys occur 

 between the central axis of the arches and their vertical 



