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which lived in the Lower Siluiian seas have left their remains in volcanic ashes, 

 which are icterstratified with Llandeilo shales and niuJs which were accumulated 

 in those seas, and which are themselves overlaid hy sheets of lava, which must 

 have poured into the sea. The same history attaches to the Corndon country, 

 which you have visited from Church Stretton. The Corndon mountain is a 

 mass of igneous rock which was erupted through the overlying Llandeilo rocks, 

 and on the western flanks of the Stiper stones are bands of volcanic ashes which 

 are interstratified with marine strata ; and these ashes were probably ejected 

 from a volcanic vent which reared its crest above the Llandeilo seas. It is not, 

 however, to direct earthquake and volcanic action that we owe the peculiar 

 scenery of Builth, and the Corndon and Stiperstone districts. It is to denudation 

 that we owe the general moulding of the features of this country ; to 

 denudation before the submergence of Wales beneath the glacial sea ; to 

 denudation during that submergence beneath an ice-traversed sea ; and to sub- 

 aerial denudation by glacial action, by frosts, snows, rains, and rivers, which 

 has been going on for ages since the land has been elevated as it is now. 



As there may be some present who may not understand the broad sense in 

 which the term rfenwda^i'on is used by geologists, I may be excused if I explain 

 that it implies the removal of portions of soUd rocks in situ, and the transport 

 of their materials by various agents into estuaries, the bed of the sea, or lakes, 

 by brooks, rivers, floating icebergs, or any other method by which those materials 

 were moved from the parent rocks and carried as boulders, pebbles, silt, and 

 mud, to accumulate as new strata perhaps hundreds of miles from the original 

 rock mass from which they were abraded. It applies also to the action of the 

 waves upon a coast Une. Subai'rial denudation is applied chiefly to those 

 subaerial agents such as glaciers, frosts, snows, rain, rivers, brooks, and springs, 

 forces which are always at work moulding the surface of the earth and subaerially 

 denuding its crust. 



All the great sedimentary rock masses which constitute the crust of the 

 earth, whether Cambrian, Silurian, Carboniferous, or Tertiary deposits, are the 

 result of the denudation of some pre-existing rock surface, the materials of which 

 were spread beneath the sea. We have abundant evidence throughout "Wales 

 of the action of ancient igneous and volcanic forces ; but this is as nothing 

 compared with the evidence we obtain respecting the forces of denudation 

 acting throughout incalculable epochs. 



The geologist who explores in "Wales and Siluria will do well to note the 

 effect of powerful volcanic forces during the Lower Silurian periods, and the 

 evidences of the fracture, dislocation, and upheaval of the Lower Silurian rocks ; 

 but he should note also the denudation of those strata before the deposition of 

 the Tipper Llandovery rocks, a denudation probably of Lower Llandovery date. 



The hardness of the lava beds, and the softness of the Llandeilo slates, and 

 the interbedded ash beds, are the cause of the irregularities of the Builth and 

 Corndon districts. Denudation and its agents have excavated the rocks into 

 their present form. 



