560 T. Mellard Reade — British Geology. 



The only break in the series of any moment occurs between the 

 Ordovician and Silurian. There can be very little doubt that their 

 areal extension was proportionately as great as their thickness. 

 Between the Carboniferous and the Silurian there is a strong 

 unconformity in the north of England and North Wales, but this 

 is bridged over, as already explained, in Shropshire, Herefordshire, 

 and South Wales, by the Old Red Sandstone, estimated at 10,000 

 feet. The Silurian shades upwards into the Old Eed by well- 

 defined passage beds, to be seen in many sections, while the Old 

 Eed graduates into the Carboniferous in no less perfect a manner. 



The Carboniferous is an extensively developed foruiation in Britain. 

 The base of Mountain Limestone varies in thickness from 1000 feet 

 in Monmouthshire to 3000 feet in the Mendips in Somerset, while 

 in Derbyshire it is 1600 feet, in the Vale of Eden 2000 feet, and in 

 Llangollen 1200 feet thick. The Millstone Grit reaches a maximum 

 of 5000 feet, the Coal-measures of South Wales 7000 to 8000 feet, 

 and, according to Logan, reaches 10,000 to 12,000 feet in Mon- 

 mouthshire, Glamorganshire, and Pembroke. In Lancashire the 

 Coal-measures are 6600 feet, though the upper portions have been 

 denuded.^ 



If we add these figures to the Cambrian and Silurian we arrive at 

 a grand total of 55,000 feet, but it is not to be inferred that this 

 occurs in any one place. Ramsay estimates the Silurians, Lower 

 and Upper, the Old Red, and Coal-measures in South Wales at from 

 20,000 to 33,000 feet (Mem. of Geol. Survey, vol. i, p. 316). 



It is of these enormously developed rocks that the typical mountain 

 areas of England and Wales are composed. The question may well 

 suggest itself to us whether there is not here a relation of cause and 

 effect. We have seen that with the exception of the break between 

 the Ordovician and Silurian there has been continuous deposit of 

 sediment from the beginning of the Cambrian to the end of the 

 Silurian, and still further, that in the Old Red Sandstone districts 

 this sedimentation appears to have gone on almost uninterruptedly 

 to the close of the Carboniferous. It is certainly a striking fact that 

 a portion of the earth loaded with sediment to the vertical extent of 

 some five to ten miles should be the very spot where the earth's crust 

 has been elevated not less than four miles, according to Ramsay, 

 and where even now the basal wrecks of these former gigantic 

 elevations should reach 3000 feet above the sea-level. Consider for 

 a moment what five miles of consolidated sediment means. It is 

 equal to a load of 2000 tons per square foot on the normal crust 

 of the earth. 



This loading doubtless squeezed and shifted laterallj'^ some of the 

 deeply lying and more mobile matter underlying the normal crust, 

 until a balance of stability was attained. Then, by a process of 

 expansion, the eftect of a necessary lise of temperature of the old 

 crust and overlying deposits, the conditions were reversed : the 



^ The authorities for these statements are principally Murchison and Eamsay, 

 together with other authorities quoted by H. B. Woodward in the "Geology of 

 England and Wales." 



