Evidence from “faults.”—One plain and obvious method of showing the great extent to which the general surface of the country has been lowered by denudation is furnished, as is well known, by the way in which the inequalities of surface produced by faults or dislocations have been effaced. It is quite common to meet with faults where the strata on the one side have been depressed several hundreds—and in some cases thousands—of feet below those on the other; but we seldom find any indications of such on the surface, the inequalities on the surface having been all removed by denudation. Now, in order to effect this, a mass of rock must have been removed equal in thickness to the extent of the dislocation. The following are a few examples of large faults:
The great Irwell fault, described by Professor Hull,[[34]] which stretches from the Mersey west of Stockport to the north of Bolton, has a throw of upwards of 3,000 feet.
Some remarkable faults have been found by Professor Ramsay in North Wales. For example, near Snowdon, and about a mile E.S.E. of Beddgelert, there is a fault with a downthrow of 5,000 feet; and in the Berwyn Hills, between Bryn-mawr and Post-gwyn, there is one of 5,000 feet. In the Aran Range there is a great fault, designated the Bala fault, with a downthrow of 7,000 feet. Again, between Aran Mowddwy and Careg Aderyn the displacement of the strata amounts to no less than from 10,000 to 11,000 feet.[[35]] Here we have evidence that a mass of rock, varying from one to two miles in vertical thickness, must have been denuded in many places from the surface of the country in North Wales.
The fault which passes along the east side of the Pentlands is estimated to have a throw of upwards of 3,000 feet.[[36]] Along the flank of the Grampians a great fault runs from the North Sea at Stonehaven to the estuary of the Clyde, throwing the Old Red Sandstone on end sometimes for a distance of two miles from the line of dislocation. The amount of the displacement, Dr. A. Geikie[[37]] concludes, must in some places be not less than 5,000 feet, as indicated by the position of occasional outliers of conglomerate on the Highland side of the fault.
The great fault crossing Scotland from near Dunbar to the Ayrshire coast, which separates the Silurians of the South of Scotland from the Old Red Sandstone and Carboniferous tracts of the North, has been found by Mr. B. N. Peach, of the Geological Survey,[[38]] to have in some places a throw of fully 15,000 feet. This great dislocation is older than the Carboniferous period, as is shown by the entire absence of any Old Red Sandstone on the south side of the fault, and by the occurrence of the Carboniferous Limestone and Coal-measures lying directly on the Silurian rocks. We obtain here some idea of the enormous amount of denudation which must have taken place during a comparatively limited geological epoch. So vast a thickness of Old Red Sandstone could not, as Mr. Peach remarks, “have ended originally where the fault now is, but must have swept southwards over the Lower Silurian uplands. Yet these thousands of feet of sandstones, conglomerates, lavas, and tuffs were so completely removed from the south side of the fault previous to the deposition of the Carboniferous Limestone series and the Coal-measures, that not a fragment of them is anywhere to be seen between these latter formations and the old Silurian floor.”[[39]] This enormous thickness of nearly three miles of Old Red Sandstone must have been carried away during the period which intervened between the deposition of the lower members of the Lower Old Red Sandstone and the accumulation of the Carboniferous Limestone.
Near Tipperary, in the south of Ireland, there is a dislocation of the strata of not less than 4,000 feet,[[40]] which brings down the Coal-measures against the Silurian rocks. Here 1,000 feet of Old Red Sandstone, 3,000 feet of Carboniferous Limestone, and 800 feet of Coal-measures have been removed by denudation off the Silurian rocks. Not only has this immense thickness of beds been carried away, but the Silurian itself on which they rested has been eaten down in some places into deep valleys several hundreds of feet below the surface on which the Old Red Sandstone rested.
Turning to the American continent, we find the amount of rock removed to be even still greater. In the Valley of Thessolon, to the north of Lake Huron, there is a dislocation of the strata to the extent of 9,000 feet.[[41]]
In front of the Chilowee Mountains there is a vertical displacement of the strata of more than 10,000 feet.[[42]] Professor H. D. Rogers found in the Appalachian coal-fields faults ranging from 5,000 feet to more than 10,000 feet of displacement.
In the Nova Scotia coal-fields one or two miles in thickness of strata have been removed in some places.[[43]]
A great fracture runs along the axis of the Sierra Nevada for 300 miles, accompanied by a dislocation of 3,000 to 10,000 feet.[[44]]