96 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



merates made up almost entirely of fragments of different lavas (por- 

 phyrites, diabases, &c.), and more than 100 feet thick, which often 

 show little or no trace of stratification, but break up into large 

 quadrangular blocks by means of joints which cut across the im- 

 bedded boulders. These volcanic conglomerates form some of the 

 more conspicuous features of the coast to the south and north of 

 Oban. They offer many points of resemblance to [those of ' Lake 

 Caledonia.' They have obviously been derived from lava-sheets 

 that were exposed to strong breaker-action, which at the same time 

 transported and rounded blocks of granite, schist, and other crystal- 

 line rocks derived from the adjacent hills. During the intervals of 

 quieter sedimentation indicated by the fine sandstones and shales, 

 volcanic explosions still continued, as may be seen by the occurrence 

 of occasional large bombs which have fallen upon and pressed down 

 the fine ashy silt that was gathering on the bottom. 



But these earlier explosions were merely preliminary to those 

 which led to the outflow of the great sheets of lava that now 

 constitute so large a part of the hills of Lome* In the few traverses 

 which I have made across different parts of this district I have noted 

 the general resemblance of the rocks to those of the Lower Old Red 

 Sandstone of the Midland Valley of Scotland, their bedded character, 

 and the occurrence of occasional layers of stratified material between 

 them. The work of the Geological Survey, however, having now 

 nearly reached /this ground, we shall before long be in possession of 

 detailed information regarding the general character and sequence 

 of its volcanic history. 



The Cheviot Hills represent a portion of the outpourings of 

 another important and active volcanic centre of Lower Old Eed 

 Sandstone time. This tract has now been mapped by the Geological 

 Survey both on the English and Scottish sides of the Border, and 

 thus its detailed structure is well known. To the Survey maps and 

 Memoirs, and to the papers by Mr. Teall, I must content myself to 

 refer for full information. 



To the north of the Cheviot area, and possibly independent of 

 it, lie the remarkably coarse agglomerates and felsitic bosses, dykes, 

 and veins, so well exposed along the coast between Eyemouth and 

 St. Abb's Head. The occurrence of Pterygotus and other organic 

 remains in the continuation of some of these rocks inland defines 

 their geological age. To the west of them lie the granite bosses of 

 Priestlaw and Cockburn Law and the quartz-porphyries of the two 

 Dirrington Laws. In these various protuberances we not improb- 



