236 REPORT OF MEETINGS FOR 1902 



thousand feet instead of the much greater thickness I should 

 myself assign to them. To this four thousand feet must be 

 added the thickness of other rocks older than those seen at 

 the Siccar Point, known to have been removed elsewhere 

 in the south of Scotland at the time in question. This fully 

 doubles the amount. So the amount of denudation during 

 the period in question will be 8000 x 3000, i.e., 24,000,000 ; 

 ■which, on this computation, is the interval of time that elapsed 

 from the date when the Gala Rocks of the Siccar Point were 

 formed down to the time when the crumpling and denudation 

 had ceased, and the rocks of Devonian age, represented by 

 the rocks of the Cheviots and St. Abbs, began to be deposited. 



The interval of time next to be considered must also be one 

 of enormous length. Not only have we to take into account 

 the time required for the growth of enormous piles of sediment 

 and the gradual evolution of very large volcanoes, but we 

 have again to bear in mind the extensive and important 

 changes in the organic world which ensued in the interval. 

 I will not now enter into detail, but simply state that the 

 changes which ensued during this Devonian period seem to 

 me to require for their accomplishment not less than 

 100,000,000 years. 



We have not done with the tale yet: — After the period 

 in question was ended there ensued another long interval 

 during which, over large areas in the south of Scotland and 

 also elsewhere, the whole of the previously formed volcanic 

 and associated strata of Devonian age were wasted away. 

 Nobody knows what that thickness was in the south of 

 Scotland. But elsewhere it can be shown that many thousands 

 of feet of rock were denuded before the next rock, the 

 Upper Old Red Sandstone, began to be formed. If we take 

 the thickness known to have been removed in the interval 

 in question in the Pentland Hills (where," by the way, it is 

 much less than elsewhere), and set it at six thousand feet, 

 which is the interval between the highest geological horizon 

 and the lowest upon which the Upper Old Red Sandstone 

 lies, there we shall be well within the mark. 6000 X 3000 

 amounts to 18,000,000 years. 



This brings us back to the section at the Siccar Point. 

 There we have Upper Old Red Sandstone lying in an almost 



