82 Mr. J. Bryce 07i the Lignites and Altered Dolomites 



three straits or narrow channels dividing Bute into four 

 islands. From measurements made by Robert Chambers, 

 Esq., and given in his beautiful and interesting work on An- 

 cient Sea-Margins, just published, it appears that these low 

 tracts are about thirty feet above the present sea-level ; an 

 elevation very nearly the same as that of the flat region between 

 Loch Lomond and the Clyde, in which we have the additional 

 and more satisfactory evidence derived from shelly deposits. 



Another interesting feature in the structure of Bute, and 

 one intimately connected with the origin of the low tracts re- 

 ferred to above, is the terrace which surrounds almost the 

 whole island, at the height of between twenty and thirty feet 

 above high-water mark, and along which the road is conducted 

 throughout the greater part of the coast. The cliffs, which 

 in many parts rise above this terrace, are often worn into 

 caves, and bear other obvious marks of the action of the sea. 

 This terrace is no doubt the former beach. It is well-marked 

 along the opposite coast from Gourock to Largs, in the Cum- 

 brays, in Arran, and on most of the sea-lochs connected with 

 the great estuary of the Clyde. Taking this along with other 

 evidence accumulated by Mr. Smith of Jordan Hill in various 

 papers, we cannot hesitate to admit, that at a comparatively 

 recent period such a change of level has been effected in Bute 

 as to convert a detached group of islands separated by narrow 

 straits, into continuous land. 



3. The valleys just described are the boundaries between 

 dissimilar strata, the line of junction generally keeping the 

 middle of the valley, but often concealed either by marshy 

 ground or accumulations of shingle. The northern portion 

 of the island, between the Kyles on one side and Kaimes and 

 Ettrick bays on the other, is composed of mica-slate. The 

 district south of this, bounded by the Rothsay Valley, presents 

 siliceous and common clay-slate; while the portion extending 

 from this valley to that of Kilchattan is occupied by red sand- 

 stone, usually conglomerate; and finally, the southern portion 

 is composed of various rocks of the trap family erupted through 

 the sandstone and overlying it. The connexion of these strata 

 with the mainland is most intimate. The slate and sandstone 

 are the terminal portions of those great bands of sedimentary 

 strata which extend from Angus to the Clyde, being parallel 

 throughout to the granitic axis of the Grampian chain ; while 

 the erupted rocks in the south of the island are a prolongation 

 of the great outburst of the igneous formations, which, affect- 

 ing a general parallelism with the same axis, stretches from 

 sea to sea in considerable ranges, as the Kilpatrick and Camp- 

 sie Hills, the Ochills and some minor ridges in the south-east 



