176 



country to Llantlilo? and is known as the " Tilestone " in many places. We have 

 not traced the existence of this peculiar rock on the east side ot Usk. No doubt 

 it occurs from Clyt)\a to Usk and Llangibby. lioughly speaking, a quarter of 

 an inch to half an inch selvage or border ought to be run round the area now 

 coloured as Siliuian on the geological charts, and this amount taken from the 

 Old Ked. But it must not be di-awn in a continuous equal belt all round for 

 reasons wliich will presently be shown. 



FAULTS IN THE USK DISTRICT. 



These faults are much easier described than investigated. It is but blind 

 ■work, with the obstinate agi'iculture which covers every rock exposure, the 

 depth of the drift, and the soft nature of the Old Red Marls and shales ; and 

 Dr. M'CuUough and myself, in some persevering work, found it all but 

 impossible to get a clear section. 



One clearly recognised fault, and some past experience that OTir faultel 

 do not occur singly, has enabled me to lay a foundation which I trust some 

 future workman of the club wUl extend and build upon. The groat master 

 "faults "of the neighboiu-mg coal field run N.N. W. and this is parallel to the 

 course of the Usk through the district in question, from the Chain bridge right 

 down to Usk and Llanllowel. Now if we can find a fault of importance along 

 this line, commensur.ate vnth. the line of excavation of the valley, we may have 

 confidence in inferring others parallel to it ; and if we can prove even one 

 crossmg the district in a direction a little N. of East, such line wiU also give us 

 confidence in others less conspicuous in the same direction. There is a well- 

 known brook which drains the hills about Mamhilad, and which runs past 

 the Tucking Mill. The v.iUey fonned by this brook answers the purpose, and 

 it Is quite easy to prove that the rooks which border the Silurian district from 

 Panteg to Glascoed do not correspond in direction or composition with those on 

 the north side of the brook. The red and [lurple giit which takes the place of 

 the Downton Sandstone is plain enough in the hiUy meadows above Llanvihangel, 

 but it suddenly ceases and is met by soft old Ked Sandstone which comes up 

 quite abruptly against a low cliff of Ludlow rock at Cwmhu-, and the high lane 

 above Glascoed farm is excavated in Aymestry Limestone, while itself stands 

 as a ridge above ground which is blood red by the decomposition of the Old Ked. 

 Here we have plain evidence that the sudden change in the features of the 

 ground is traceable to the cause which has notched one formation deep into the 

 other : .and following up this hint we infer another fault along the line which 

 has divided the hard sandstones of Glascoed Common by a deep depression. 

 We can understand, by means of these faults, why the hard sandstones of 

 CUfigan-park and Frescoed should form so straight a terrace, and why there 

 should be such deep indentations of the border, and sudden protrusions of 

 Silurian rock among the Old Red, at Llanvrechfa on the south side of the 

 district. 



