154 



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 

 in 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 

 layers according to their affinities, while the whole was subjected to 

 pressure acting along certain axes of elevation, which raised those 

 layers into arches. 



February 26, 1852. 



COLONEL SABINE, Treasurer and V.P., in the Chair. 

 The following paper was read : — 



" On the Motions of the Iris." By B. E. Brodhurst, Esq., M.R.C.S. 

 Communicated by Thomas Bell, Esq., Sec. R.S. Received Novem- 

 ber 20, 1851. 



The observations made in this paper are distributed under three. 



