152 Geological /Society. 



broad differences in chemical composition are therefore to be 

 expected between the igneous rocks of orogenic belts and those 

 erupted in connexion with gentle subsidence. 



The Archaean plutonic rocks were intruded in close relation with 

 powerful lateral thrust, and they accordingly include no alkaline 

 types ; but the Dalradian sediments were deposited in an area of 

 tranquil subsidence, and the lavas intercalated in them are of the 

 spilitic kind, rich in sodic felspars. 



The Lower Palaeozoic formations were laid down in a geo- 

 syncline, which for a long time experienced merely a slow 

 depression, and the late Cambrian and early Ordovician eruptions, 

 situated chiefly along the borders of the area, had a pronounced 

 sodic facies. In Mid-Ordovician times there entered a certain 

 element of lateral thrust, and accordingly in the Llandeilian 

 vulcanicity the spilitic type gave place to the andesitic ; but the 

 scattered outbreaks of Bala and Silurian age often afford evidence 

 of a reversion to the earlier facies. 



Following upon the great Caledonian crust- movements there 

 was, in the Scottish Highlands and elsewhere, a copious intrusion 

 of plutonic magmas, all of ' calcic ' as contrasted with alkaline 

 types. The same characteristic belongs to the igneous rocks of 

 the Lower Old Eed Sandstone, which were extruded and intruded 

 in connexion with the later Caledonian folding, while the 

 country was still in a condition of stress. With the dying-out 

 of this stress a more alkaline facies supervened, and the Lower 

 Carboniferous igneous rocks of Scotland, though developed largely 

 in the same synclinal folds as the preceding series, present a strong 

 contrast in petrographical characters. They indicate a certain 

 richness in soda, and this feature becomes more pronounced, until 

 it culminates in the Permian of Ayrshire and East Fife in highly- 

 alkaline rock-types. 



In Southern England, remote from the main Caledonian dis- 

 turbance, the Devonian and Carboniferous lavas are of the same 

 spilitic type as those of the early Ordovician. Later, this part of 

 the British area was involved in the Hercynian crust-movements, 

 which were accompanied by the intrusion of the Cornish granites 

 and their satellites. 



In Mesozoic times our country experienced no orogenic dis- 

 turbance of a pronounced type, and there was a prolonged cessation 

 of igneous activity. The Tertiary Era introduced a new factor in 

 the form of very extensive plateau-faulting, bearing no relation to 

 the structure of the countiy. This movement, generally of the 

 nature of subsidence, affected a vast area, of which Northern 

 Britain is only a small fraction, and was attended by igneous 

 action on the same extensive scale. The mechanism of extrusion 

 and intrusion differed in important features from that illustrated 

 by the Palaeozoic eruptions. The Tertiary igneous rocks, as a 

 whole, are decidedly, though not strikingly, rich in soda ; but 

 this alkaline character is lost in the neighbourhood of isolated 

 centres, where there is evidence of locally-developed stresses of 

 an acute type. 



