ANNIVEESART ADDRESS OF THE PRESIDENT. 93 



region wliich I am now describing, in the tract that surrounds 

 the granite boss of the Cheviot Hills. The dykes consist there of 

 granite, felsite, quartz-porphyry, and porphyrite. Of the latter rock 

 Mr. Clough mapped no fewer than forty dykes. He noted, moreover, 

 that not only these, but the dykes of felsite and quartz-porphyry 

 tend to point, in a general way, towards the granite, as if that 

 were the great centre from which they radiate.^ 



Volcanic activity had entirely ceased in * Lake Caledonia ' long 

 before the last sediments of the Lower Old Eed Sandstone had been 

 laid down. The great cones of the Ochil Hills, for example, sank 

 below the waters of the lake in which they had so long been a 

 conspicuous feature, and so protracted was the subsidence of the 

 lake-bottom that the site of these volcanoes was buried under 8000 

 or 9000 feet of sandstones and conglomerates, among which no trace 

 of any volcanic eruptions has yet been found. The sagging of the 

 terrestrial crust over an area from which such an enormous amount 

 of volcanic products had been discharged, would doubtless be a 

 protracted process. Long after the subsidence of the lake-bottom 

 and the accumulation of its thick mass of sediments, after even the 

 entire effacement of the topography and the deposition of the thick 

 Carboniferous formations over its site, the downward movement 

 showed itself in the production of the gigantic north-east faults, and 

 the sinking of the Carboniferous rocks for several thousand feet. 

 Those dislocations, as was natural, have run through the heart of 

 some of the volcanic groups, carrying much of the evidence of these 

 ancient volcanoes out of sight, and leaving us only fragments from 

 which to piece together the records of a volcanic period which is by 

 no means the least interesting in the long volcanic history of this 

 country. 



The limits of this Address forbid me to enter upon an adequate 

 description of the other volcanic regions which I have above 

 enumerated, with the view of showing that the time of the 

 Lower Old Eed Sandstone was eminently marked by volcanic 

 activity in the region of the British Isles. A rapid glance at 

 some of their more salient features, however, may help to deepen 

 the impression which we receive from a study of the phenomena of 

 Central Scotland. 



In the year 1878, I called attention to the evidence for the 

 existence of contemporaneous volcanic rocks in the Old Eed Sand- 



^ Geol. Surv. Mem. on the Clieviot Hills, p. 28. 



