116 Rev. J. Clifton Ward — Geology of the Lake District. 



distance from the actual area of a delta means a decreased rate of 

 deposition, we may assume perhaps that the 10,000 feet of Skiddaw 

 Slates were laid down at the rate of -^ of an inch per annum, and 

 this would give a period of six million years for the formation of 

 the whole series. If, however, we take the rate at ■& instead of -gV, 

 the period is reduced to less than 1-| million. 



Volcanic Period. — No reliable estimate can be made of the rate of 

 deposition of volcanic material. The length of the periods of repose 

 between successive eruptions appears to be very variable. In this 

 particular case, however, there is nothing to lead one to suppose that 

 the eruptions were not tolei'ably continuous. There seem to be no 

 great breaks in the series, and, for aught we know to the contrary, 

 the whole thickness of some 12,000 feet may represent continuous 

 and violent action. Hence I am inclined to put the rate of accumu- 

 lation at not less than -^ of an inch per annum, which gives a pei'iod 

 of 1^ million years. If we take \ as the rate — as Mr. Morgan has 

 assumed in the case of Etna — the length of the whole period is 

 reduced to 720,000 years. The gap between this and the next 

 period might be put down at \ a million of years, perhaps some 500 

 feet having been removed at the rate of 1 foot in every thousand 

 years, the denudation acting upon a sinking area. 



Upper Silurians of Lake District. — The same remarks may be 

 made on this series of sedimentary deposits as on those of the 

 Skiddaw Slates. Although in great part they must have been 

 formed in tolerably shallow water, yet might the deposition have 

 gone on more or less outside the limits of a delta proper. Taking 

 therefore the rate of deposition at -3V of an inch per annum, a thick- 

 ness of some 14,000 feet of beds may represent a period of about 

 8^ million years. 



Old Bed Period. — Here we have not facts of deposition to calculate 

 from in this district, but those of denudation. What length of time 

 must have elapsed between the close of the Upper Silurian and the 

 commencement of the Carboniferous, to allow of the removal of 

 probably more than 20,000 feet of rock, so that the Mell Fell con- 

 glomerate could be deposited transgressively upon both Volcanic 

 Rocks and Skiddaw Slates ? The length of time must have been 

 great indeed, for the extensive denudation necessitates also a great 

 amount of continued elevation to allow of the deeply buried strata 

 coming within the range of the denuding powers. A planing action is 

 required to cut off the rising land along the main axis of upheaval ; 

 and the sea alone can supply us with this. The work, in fact, would 

 be most efficiently done if we could suppose the elevation to be going 

 on but very little faster than the sea could gnaw and eat the land 

 down. Now are we warranted in assuming that the rate of denu- 

 dation under such circumstances would be greater than in the case 

 of a tract of country lowered mainly by subaerial agencies ? I 

 think we are. Prof. G-eikie has calculated that the area of country 

 drained by the Mississippi is being lowered at the rate of one foot in 

 6000 years, while that drained by the Po is being lowered at one 

 foot in 729 years ; the Ganges, Hoang Ho, Rhine, Danube, and Nith 



