part 1] AXXIYEESARY ADDllESS OF THE PEESIDEXT. IxxXV 



The volcanic rocks of the Lower Old lied Sandstone belong 

 everywhere to the andesitic series, ranging from basalts to rhjolites, 

 but with p3'roxene-andesites as the dominant types. No general 

 law of sequence is apparent. The plutonic rocks are mostty 

 granites and quartz-diorites, tending, however, in Argyllshire to 

 monzonitic types. The dyke-rocks are comparable with those 

 found as satellites of the Newer G-ranites, and the petrographical 

 characters of the suite as a whole shov/ that the influence of the 

 Caledonian disturbance was still felt. It is to be regretted that 

 we possess no detailed petrographical knowledge concerning the 

 feeble and sporadic revival of vulcanicity in Upper Old Red Sand- 

 stone time, when this influence was presumably exhausted. The 

 record of this episode is preserved in Caithness and the Orkneys, 

 in Arran, and in the neighbourhood of Limerick, and the rocks of 

 the tirst-named district are, according to Dr. Flett, basalts of 

 alkaline affinities. 



Setting aside these scattered outbreaks, the next important 

 manifestation of igneous activity in Northern Britain began early 

 in Carboniferous time. The conditions were now wholly changed. 

 All folding related to lateral thrust had long ceased, and a general 

 subsidence w^as in progress. The change was reflected in the com- 

 position of the igneous magmas now brought into action, which 

 were of an alkaline character, at first but moderately pronounced 

 though becoming progressively sti'onger. 



The subsidence was not uniform, and the vulcanicity related to 

 it had a distribution sufficiently significant. The chief theatre of 

 activity was the Midland Valley of Scotland, prolonged to what is 

 now the Mull of Kintyre ; and the Carboniferous lavas and tuffs 

 are thus found in close proximity to the very different volcanic 

 rocks of Old Red Sandstone age along the borders of the same 

 tract. Another volcanic belt was situated on the other side of the 

 Southern Uplands, coiTesponding roughly with the axis of the 

 Old Red Sandstone basin of Cheviot. There were also more iso- 

 lated centres of activity, one as far away as Limerick ; but I shall 

 deal with the matter briefly, taking the Midland Valley as a typical 

 area. 



One point of interest brought out by the detailed mapping is 

 the gradual shifting of the theatre of activity, in general from east 

 to west. In the east volcanic rocks are practical^ confined to the 

 Calciferous Sandstone Series, and later igneous action took the form 



