THE BLACK HILL OF EARLSTON 55 



subsidence of the land, thousands of feet of mud, sand, and 

 silt, with foraminiferal ooze in the deep water, and beds of 

 coal in the marginal areas of the sea around the deltas, were 

 gradually deposited one over the other, until the time came 

 for yet another movement upwards, and another return to 

 continental conditions. It was under these latter geographical 

 conditions that the New Red strata were formed, vestiges of 

 which occur in the form of stained rocks (or even of actual 

 outliers, as the two I have before referred to at Dunbar) 

 over so large a portion of Scotland. Later still came renewed 

 subsidences, the first of which brought in first the Rhsotic 

 Rocks, and then the Jurassic beds, both of which may well 

 have been here at one time. 



So the story might be repeated again and again — one geo- 

 graphical phase gradually taking the place of another, for 

 hundreds of millions of years, from then till now. 



At an early period in the history of the Carboniferous 

 Rocks, volcanic action commenced at several independent 

 centres in what is now the South of Scotland. None of the 

 volcanoes referred to ever attained to any great size ; nor 

 does it appear that any of them lasted long. The centres of 

 eruption shifted from place to place, as time went on, and 

 they continued to do so throughout nearly the whole of 

 Lower Carboniferous times. The earlier volcanic products, 

 in most cases, were basic in composition, so that the lava 

 streams were mostlj' basalts. 



But in each latter phase of the history of a volcano, it is 

 commonly the case that the materials erupted differ more or less 

 from those given forth during the earlier period. A volcano 

 which begins by pouring out basalt lavas commonly does, at 

 a later period, give forth a fluid rock in which much more 

 potash and silica occur, (I have suggested that this might be 

 due to the fact that the gradual enlargement of the volcanic 

 focus leads to some granite being reached. This, being once 

 more gradually reduced to the fluid state, is expelled in the 

 direction of the surface.) When a volcano reaches maturity, 

 the pressure required to expel the fluid rock to the surface 

 is often also sufficient to enable it to make its way in amongst 

 the rocks below the surface to some distance from the focus. 



