352 



TEE POPULAR SCIENCE II ONTELY.— SUPPLEMENT. 



One plain and obvious method of showing the 

 great extent to which the general surface of the 

 country has been lowered by denudation is fur- 

 nished, as is well known, by the way in which the 

 inequalities of surface produced by faults or dis- 

 locations 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 that on 

 the other, but we seldom find any indications of 

 such on the surface, the inequalities on the sur- 

 face having been all removed by denudation. 

 But in order to effect this a mass of rock must 

 have been removed equal in thickness to the ex- 

 tent of the dislocation. The following are a few 

 examples of large faults : 



The great Jrwell fault, described by Prof. 

 Hull, 1 which stretches from the Mersey west of 

 Stockport to the north of Boltou, has a throw of 

 upward of 3,000 feet. 



Some remarkable faults have been found by 

 Prof. Ramsay in North Wales. For example, 

 near Snowdon, and about a mile east-southeast 

 of Bcddgelert, there is a fault with a down-throw 

 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 down-throw of 

 ^,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. 5 Here we have evidence that a mass of 

 rock, varying from one mile to two miles in ver- 

 tical 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 

 upward of 3,000 feet. 3 Along the flank of the 

 Grampians a great fault runs from the North Sea 

 at Stonehaven to the estuary of the Clyde, throw- 

 ing the Old Red Sandstone on end sometimes for 

 a distance of two miles from the line of dislocation. 

 The amount of the displacement, Prof. Geikie 4 

 concludes, must be in some places not less than 

 5,000 feet, as indicated by the position of occa- 

 sional outliers of conglomerate on the Highland 

 side of the fault. 



The great fault crossing Scotland from near 



1 " Memoirs of the Geological Survey of Lancashire," 

 1862. 



2 " Memoirs of the Geological Survey of Great Britain," 

 vol. iii. 



3 " Memoir to Sheet 32," Geological Survey-map of 

 Scotland. 



4 Mature, vol. xiii., p. 390. 



Dunbar to the Ayrshire coast, and which sepa- 

 rates 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, 1 to have in some places 

 a throw of fully 15,000 feet. This great disloca- 

 tion is older than the Carboniferous peiiod, 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 enor- 

 mous amount of denudation which must have 

 taken place during a comparatively limit ed geo- 

 logical 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 southward 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 scries and the Coal- 

 measures, that not a fragment of them is any- 

 where to be seen between these latter formations 

 and the old Silurian floor." This enormous 

 thickness of nearly three miles of Old Red Sand- 

 stone must have been denuded away during the 

 period which intervened between the deposition 

 of the Lower Old Red Sandstone and the accumu- 

 lation 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, 2 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. 



Faults to a similar extent abound on the Con- 

 tinent and in America, but they have not been 

 so minutaly examined as in this country. 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. 3 



In front of the Chilowee Mountains there is a 

 vertical displacement of the strata of more than 



1 " Explanation to Sheet 15," Geological Survey -map of 

 Scotland. 



2 Jukes's and Geikie's "Manual of Geology," p. 441. 



3 "Geology of Canada," 1863, p. 61. 



