394 MR. H. H. THOMAS AND PROF. 0. T, JONES ON THE [Sept. I9I2, 



sandstones of No. 6, and if the Solva Beds immediately succeed the 

 purple sandstones of Caerbwdy, it is possible that No. 5 represents 

 the basal beds of the Solva Series, On the other hand, as far as 

 their lithological characters are concerned, they might equally well 

 represent the upper portion of Hicks's Caerfai Group, the difference 

 in colour of the sediments being comparatively unimportant. 



With regard to the position of the Ford Beds, it might be sug- 

 gested that they belong to the Lingula Flags or to the Arenig, both 

 of which formations occur in the vicinity ; but they are quite 

 distinct lithologically from both, and it is considered likely that 

 they are older than either. 



They can hardly be compared with the Lower or Middle Solva 

 Beds, as developed in the St. David's district ; but they bear con- 

 siderable resemblance to the Upper Solva Beds, more especially 

 those of Penpleidiau, which also contain much iron-pyrites and have 

 the same tubular markings, like annelid borings. 



Of all the Lower Palaeozoic formations with which we are 

 acquainted, the Upper Solva Beds, near their junction with the 

 Menevian, offer the greatest similarity to the Ford Beds, and they 

 are provisionally assigned to that horizon. It is worth noting in 

 this connexion that the pre-Cambrian mass of Pointz Castle is 

 faulted against fossiliferous Menevian deposits to the south of 

 Loch-faen. Fossiliferous Menevian beds also occur on the southern 

 flank of llhindaston Mountain, near Ferny Glen. 



The objection to this view is the apparently unfossiliferous 

 nature of the Ford Beds, which somewhat sharply contrasts them 

 with the Upper Solva Beds of the St. David's district. 



Whatever view is ultimately adopted as to their horizon, it seems 

 clear that a stratigraphical break must occur in the district that we 

 have surveyed, for there is little, if any, trace of the Lower and 

 Middle Solva Groups, which are of great thickness near St. David's. 

 If the Ford Beds be assigned to a higher horizon, the break becomes 

 correspondingly greater. 



From the behaviour of the outcrops it is considered that any 

 rational system of faulting is inadequate to account for the strati- 

 graphical relationship of the Ford Beds to the lower horizons. 



[b) Their Relation to the Pebidian and Dimetian. 



On account of the nature of the country which we are describing, 

 the mapping had to be carried on very largely by utilizing the 

 indications as to the nature of the subsoil furnished by debris in 

 hedges and on fields ; and a few words may be added on the probable 

 correctness and value of a map of a complicated district constructed 

 in this manner. 



Experience has shown us that in this part of the country, as well 

 as in other parts of Pembrokeshire where the rocks are scantily 

 exposed, a map can be constructed by this method which attains 

 almost as high a degree of accuracy as if exposures were much 

 more frequent, and is of far greater accuracy than if the solid 



