750 BEPOKT— 1893. 



■which two are visiWe at the surface in the road leading by Cinder Hill Brickyard, 

 and -which are laid down on the maps of the Geological Survey. 



There are three points in connection with this series of ' faults ' which may he 

 especially interesting to this Association, and which were illustrated on the section 

 displayed on the screen. 



The first point of interest is that whilst the ' throw ' of the ' fault ' in the Per- 

 mian rocks at the surface only amounts to 76 feet, in the Coal Measures below the 

 ' throw ' amounts to 340 feet, a very much larger amount; and this shows that 

 there was an enormous movement prior to the deposition of the Permian strata, 

 and that there was a further small movement on the same lines after the deposi- 

 tion of the Permians. Both these movements took place when the rocks which 

 they affected had obtained their full degree of induration, and it is obvious that 

 the interval which elapsed between the two movements must have marked a long 

 period of geological time. 



The second point which is noticeable about this series of ' faults ' is the great 

 lateral movement of which it gives evidence. AVhilst the ' throw ' of the ' fault ' 

 on the slide or hade amounts to 340 feet, and the vertical displacement amounts 

 to 210 feet, the horizontal movement amounts to 270 feet, showing enormous 

 lateral movement. 



A third point may also be noticed, which is, that the main fault, though a large 

 one, practically makes no difference in the position of the zone of strata through 

 which it cuts. If the coal seana on the left of the section shosvu be continued at 

 its normal dip across the faulted ground, the position of the large ' fault,' it was 

 .seen that it practically meets the coal on the upcast oi" eastern side of that 

 * throw.' The rapid dip of the strata on the westward of the ' fault ' line is thus 

 merely the slipping of the different beds along the fractural edge of the fault, 

 owing to the horizontal movement already indicated ; and the fractures, the 

 alterations in inclination, are the result mainly of horizontal and not of vertical 

 movements. 



10. On the Base of tJie Camhrian in, Wales. Bij H. HiCKS, M.D., 



F.B.S., F.G.S. 



If there is, as has been maintained by the author and others, a very marked 

 unconformity at the base of the rocks usually classed as Cambrian in Wales, the 

 evidence furnished by an examination of those basal beds which indicate shore 

 condition is of the utmost importance. The author, therefore, in this paper gives 

 a summary of the results bearing on this question which he has obtained iu his 

 examinations of these rocks in Wales. 



Pembeokeshtee. 



St. David's. — The basal beds are exposed on the north and south sides of the 

 pre-Cambrian Axis. Where faults do not intervene the lowest beds are rough 

 conglomerates from GO to 150 feet in thickness, in which pebbles over a foot in 

 diameter are very frequently met with. The matrix and pebbles vary constantly, 

 as they rest on different parts of the pre-Cambrian Axis, and there is the clearest 

 evidence of an unconformity between the conglomerates and the highest beds of 

 the Pebidian exposed in this area. The overlying beds, which are grits and sand- 

 stones, are ripple -marked, and show other proofs of having been deposited in 

 shallow shore-water. The author has recently re-examined the basal beds in this 

 area, and has accumulated additional evidence in support of the above view. 



Samsey Island.— The Cambrian conglomerates here rest on pre-Cambrian 

 felstones and breccias. The pebbles are mainly well-rolled fragments of felstones 

 cemented together by a felsitic matrix. Pebbles of quartzite and other materials 

 are occasionally found, but the main amount of the material was undoubtedly 

 derived from the rocks immediately underlying the conglomerates. The under- 

 lying rocks had undergone the marked changes now visible in them before the 

 fragments in the conglomerates had been broken off. 



