Vol. 65.] ANNIVERSAEY ADDRESS OF THE PRESIDENT. CXV 



In our own islands the unconformity in question is extreme, the 

 Armorican chain had been folded up and a great part of it levelled 

 to the sea before the deposition of the Permian : the events de- 

 scribed by Ramsay in South Wales l belong to this interval, and 

 the great folds of northern England, which rose like a ' parma ? 

 in front of the Armorican chain, were also completely destroyed 

 before the Lower Permian age. Thus the red marls and sands of 

 this system rest on the Carboniferous Limestone, just north of the 

 locality where the Carboniferous System attains its greatest thick- 

 ness in these islands, no less than 10,000 or 20,000 feet of Coal- 

 Measures, Millstone Grit, and Lower Carboniferous rocks having 

 been folded up and removed in pre-Permian times. 



On the opposite side of the Pennine chain the Magnesian Lime- 

 stone rests on the Carboniferous Limestone over the region extending 

 from Richmond to Darlington ; but here the thickness of the under- 

 lying series is less, and the denudation did not amount to more than 

 5000 or 6000 feet. 



The extent to which the Armorican chain and its outlying ranges 

 suffered from denudation, prior to the deposition of the Permian, 

 is comparable to that which has affected the Alps and the Himalaya 

 of our own time, and we can scarcely doubt that the interval of 

 time which separates the Permian from the Carboniferous is as 

 great as, if not greater than, that which has elapsed since the Alpine 

 foldings of the Miocene Epoch. If expressed in terms of sediment, 

 it might amount to 30,000 or 40,000 feet. 



In our islands there seem to be at least three other unconformities 

 of the first magnitude, (1) that associated with the Caledonian move- 

 ments, (2) another immediately antecedent to the Cambrian, and a 

 third to the Torridonian. If we include the pre-Huronian and pre- 

 Animikian unconformities of North America, we raise the total to 

 six, and if we include those recorded by Sederholm to nine. 



Taking six as the probable number of great unconformities, and 

 regarding them as of equal value, their duration would be repre- 

 sented by 18 to 24 thousand feet of sediment, the equivalent of 

 18 to 24 millions of years. This, added to the 51 millions pre- 

 viously found, would give a total of 69 to 75 millions. 



There are many minor unconformities related to regressions 

 and transgressions of the sea, as well as other interruptions 

 in the deposition of sediment which must represent a lapse of 

 stratigraphically unrecorded time. These are difficult to evaluate : 

 to obtain a round number we might assign 5 millions of years 

 1 Mem. Geol. Surv. vol. i (1846). 



