416 ME. A. WIDE ON THE LLANDOVEEY AND [Aug. I9II, 



Of the particular district with which I now propose to deal he 

 has much to say. He describes very fully the Welshpool Dyke, 

 and notes its apparent effect upon the strike of the neighbouring 

 rocks, which is generally north-north-east and south-south-west, the 

 direction of the axis of the dyke. One very interesting passage 

 is worthy of quotation : referring to the ridges in the Upper 

 Park of Powis Castle, he says (op. cit. p. 290) : — 



' The dislocation in the higher ridge of the Upper Park changes its 

 direction to east-north-east and west-south-west, according with that of the 

 Breidden Hills, and it is therefore possible that the dyke of Welshpool and 

 the lines of disturbance immediately proceeding from it, are only slight aber- 

 rations from a line of eruption of which the Breiddens was the chief focus ; 

 it is, however, to be observed that the disturbed line of Welshpool is parallel 

 to the strike of the volcanic ridges of Corndon, whilst the Breidden Hills.... 

 cut through the stratified deposits in a direction which diverges 45° from those 

 parallels.' 



In dealing with the stratigraphical succession of the area, he 

 recognizes two divisions only : — 



(1) An Upper Silurian division, consisting chiefly of dull grey shales con- 

 taining occasional spherical concretions which he compares with the septaria 

 of the London Clay. Owing to the absence of limestone-bands, he makes no 

 attempt to differentiate between the Wenlock and the Ludlow. 



(2) A Lower Silurian division, which he calls Caradoc Sandstone and 

 Llandeilo Flags. It consists of impure sandy limestones in the upper beds. 

 with conglomerates, hard grits, and mottled red-and-grey shales beneath. He 

 describes sections at Powis Castle, Moel-y-garth, and the Gaerfawr, the first 

 of which he illustrates by means of a diagram. With regard to the latter 

 section he says (o£>. cit. p. 306) : — 



' This red conglomerate, overlying the whole system of the grey sand- 

 stones of the Gaerfawr, proves that the similar red conglomerate and gritty 

 beds of Welshpool, Powis Castle, the Quakers' Burial Ground, and other 

 places which rise from beneath the overlying mudstones or Upper Silurian 

 rocks, form the upper strata of the Caradoc Sandstone or Lower Silurian 

 rocks.' 



It is now possible to differentiate between the Wenlock and the 

 Ludlow ; and it can also be shown that, instead of forming the upper 

 strata of the Caradoc Sandstone, the lied Conglomerate Series lies 

 uneonformably upon them. It is, therefore, necessary to remove a 

 part of the Conglomerate and associated beds from Murchison's 

 Lower Silurian, and to establish a Llandover} T group in the area. 



In 1850 a geological map of the district, executed hy W. T. 

 Aveline & H. W. Bristow, with A. C. Ramsay as Local Director, 

 was published by the Geological Survey, together with a section. 

 For purposes of mapping, Murchison's grouping of the rocks was 

 adopted. Wide areas are mapped as Wenlock Shale, which in 

 reality are occupied by Ludlow Beds. The dividing-line between 

 Upper and Lower Silurian is drawn at the base of the upper shale- 

 beds, so that part of the Llandovery is mapped along with the 

 Wenlock, while the lower portion is mapped with the Upper 

 Ordovician strata. 



In 1866, ' The Geology of North Wales ' was published by the 



