Royal Society. 389 



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 

 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 Grampians. 

 The granite of Cruachan and Ben Muich Dhui interfere with the 

 regularity of the foliation of this district, and the lines are thrown 

 to the north by the granite of Aberdeenshire : 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 Coryaraick. This plane forms the southern 

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

 entirely of gneiss, bounded on the north by a band of vertical folia- 

 tion which runs about N. 30° E. from Glen Finnan through the 

 middle of Rosshire 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 unconformable to that hitherto described, but agrees with that of 

 the Island of the Lewis, forming a series of arches which run about 

 N.W. 



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

 lands, they would all nearly converge between Lough Foyle and 

 Lough Swilly 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 axes of the arches are generally found 

 iu high land, and the principal valleys occur between the central 

 axes of the arches and their vertical boundaries. Thus the main 

 physical features of the Highlands are connected with, the foliation 

 of the gneiss and schists ; but the granites and porphyries which 

 have broken through those rocks, and disturbed the regularity of the 

 foliation, have also greatly modified the surface of the country. 



The contortions of gneiss and schists being unaccompanied by 

 fracture, must, the author considers, have been produced when the 

 matter of those rocks was semi-fluid : in this state the mineral in- 

 gredients appear to have separated and re-arranged themselves in 



