74 



stone, capped by Millstone grit, is distant about five miles from the edge of the 

 northern rim of the AVtlsh coalfield, aud is separated from the mass of that great 

 coal Sold by the denudation of the intervening valley of the Usk, the valley 

 itself running along a line of fault, which has upheaved the Old Eedhills of the 

 Black Mountains (with Pen Cerrig Calcli on their sximmit), on the north bank of 

 the Usk, and depressed the strata on the south. Pen C'crrig Calch is 2,200 feet 

 above the sea, and is uplifted several hundred feet above the limestone with 

 ■which it was once continuous, and which ranges on the other side the vale of 

 Usk. It is impossible to visit this outlier without being impressed with the fact 

 of the dislocation of the earth's crust along the Usk valley, while denudation 

 has also been cari'ied on upon a grand scale ; and some of the relics of which are 

 stiU piled in boulder clays, and old river beds, along the Hanks of the hills. 



It is not only wdth the nearer outliers that the geologist learns to connect 

 the Mountain limestone and the overlying Blillstone grit. We feel sure that 

 these rocks were formerly spread in a continuous series over the county of 

 Hereford to the distant Olee-hills of Shropshire. Nothing appears to me more 

 ridiculous than to suppose that the Clee-hill Mountain limestone, or the outliers 

 of North "Wales, such as the Ormes-head, were little isolated coral reefs accu- 

 mulated in particular spots in the mountain limestone sea. The Millstone grit 

 which overlies them should be sufficient to demolish such ideas, for allowing 

 that isolated coral reefs did grow upin the mountain limestone sea, we cannot 

 understand how the millstone grit could have been deijosited in such an accom- 

 modating manner above every outlier by particular and jieculiar currents, which 

 spread their strata over the coral islands, and adapted their flow to such widely- 

 distant and separated areas as are those of the Little Orme district, near 

 Llandudno, the Titterstone Clee, and Pen Cerrig Calch, near Crickhowell. 



There are few geologists, however, who doubt the former great extension 

 of all the Carboniferous deposits, and of the Old Red sandstone, over areas 

 from which they have in later times been denuded. The question I draw 

 attention to more particularly on this occasion is, lohen they were denuded, and 

 how the present surface configuration of this country has arisen ? Notwith- 

 standing the evidences of volcanic action,. as displayed in the eruption of masses 

 of igneous rock among the Carboniferous deposits at the Titterstone Clee, and 

 during Old Bed and Silurian times ; notwithstanding the evidence we possess of 

 considerable earth movements, movements of elevation and depression, we still 

 arrive at the conclusions that Hutton enounced when he said that "the moun- 

 tains have been chiefly formed by the hollowing out of the valleys, and the 

 valleys have been hollowed out by the attrition of hard materials coming from 

 the mountains." Of course one of the principal agents of denudation is the great 

 abrading power of the sea. We see everywhere loss of land along coast lines, 

 and when we see this we behold the effect of marine forces which have been in 

 work throughout all ages. Without doubt ancient continents have been 

 eroded by waves acting on their coasts, but the action jf breakers and waves 



