178 PROCEEDINGS OF THE GEOLOGICAL SOCTETY. 



altogether from gravitation, there was first an extensive separation of 

 the more basic constituents, such as the ferro-magnesian minerals 

 and ores, and the lavas which came off at that time were heavy 

 and basic basalts, and*even picrites. The removal of these elements 

 left the magma more acid, and such rocks as andesites were poured 

 out, until at last the deeper intrusive sills, dykes, and bosses 

 became thoroughly acid rocks, such as felsite, quartz-porphyry, and 

 granite, while if any superficial outflow took place it was such 

 a rock as pitchstone. It is not my purpose at present to discuss 

 the probable cause of this order of appearance, but to endeavour 

 to lay a basis of ascertained facts on which the discussion may 

 proceed. 



12. A further interesting fact appears from the volcanic history 

 which we have been tracing. On the whole, we may say that each 

 eruptive period witnessed the same sequence of change from basic 

 to acid lavas. And as the successive jDrotrusions took place within 

 the same circumscribed region, it is evident that in some wa}" or 

 other, during the long interval between two periods, the internal 

 magma was renewed as regards its constitution, so that when 

 eruptions again occurred they once more began with basic and 

 ended with acid materials. This cycle of transformation is admirably 

 exhibited in Central Scotland, where the porphyrites of the Old Eed 

 Sandstone with their felsite sills are followed by the basic lavas of 

 the Carboniferous plateaux, succeeded in turn by the porphyrites, 

 trachytes, and acid sills of that series. VThen the puy eruptions 

 ensued, the magma had ..once more become highly basic. 



That the true explanation of these alternations is of a complex 

 order may be inferred from the exceptions which occur to the 

 general rule. I have alluded to the Snowdon region, where the 

 acid rhy elites are followed by more basic andesites, and there 

 the sills are also more basic than the superficial lavas. In the 

 Arenig and Cader Idris country the sills are likewise more basic 

 than the bedded lavas. .Among the Carboniferous puys of the basin 

 of the Firth of Forth, the sills are not sensibly more acid than 

 many of the superficial basalts, and they even include such rocks 

 as picrite. Possibly in this last-named region we see an arrested 

 sequence, the volcanic protrusions having for some cause ceased 

 before the general uprise of the more acid magma. 



13. There is one final group of facts which seems to me deserv- 

 ing of attention. In the great sills and bosses, particularly those 

 of basic composition, the so-called ' contemporaneous veins ' are of 



