308 A. GEIKIE ON THE SUPPOSED 



specially well developed and marked off from each other on the coast- 

 line south of St. David's. They may also be studied at Castell, on 

 Eamsey Sound. 



One of the most interesting lithological features in these strata is 

 the presence in them of diffused volcanic dust and of layers of fine 

 tuff. Some of the red shales are full of this material, which here 

 and there is gathered into the thin seams or ribs of which the micro- 

 scopic characters have already been described. This diffused vol- 

 canic detritus marks, no doubt, the enfeebled discharges of fine dust 

 towards the close of the volcanic episode in the Lower Cambrian 

 period at St. David's. It would be difficult to find an instance of a 

 more perfect transition from a series of thoroughly volcanic masses 

 into a series of ordinary mechanical sediments. 



It is further to be noted that in this zone, as discovered by Dr. 

 Hicks, well-preserved specimens of Lingulella primceva occur* 

 There can, therefore, be no doubt as to the place of the strata in the 

 geological record. The red sandstones and shales pass up insensibly 

 into a thick overlying mass of fossiliferous purple and green sand- 

 stone, assigned by Dr. Hicks to the Harlech Group. It is not needful 

 here to pursue further the upward stratigraphical succession. 



2. Geological Stetjctitee of the Disteict. 



The existence of an anticlinal axis in the Cambrian beds at St. 

 David's is indicated upon the Geological Survey Map, which does not, 

 however, distinguish the different zones of rock in such a way as to 

 show the line of axis, or to afford data for measuring the thickness 

 of the beds. As I have already pointed out, there is not only a 

 great arch of the strata here, but the south-eastern half of the arch 

 has been inverted. As the determination of this isoclinal fold is a 

 point of fundamental importance in the structure of the district, 

 some further details regarding it may here be given (see figs. 1 & 2, 

 p. 268). 



On the west side of the district the succession of beds can be 

 followed from the headland of Point St. John without interruption, 

 along the shores of Eamsey Sound, for about a mile south-eastward. 

 The strata of sandstone and shale, traversed here and there by 

 eruptive rocks, dip in a general north-westerly direction. Hence 

 there is a steadily descending section until near Castell, where the 

 shore trends toward the south-west, and coincides with the general 

 direction of the strike of the beds. "Whether we follow the coast- 

 line round into Porth-lisky, or strike inland across the promontory 

 to that bay, we encounter a thick series of tuffs (with bands of 

 diabase and occasional intrusive elvans or veins of quartz porphyry) 

 presenting the same general dip towards W.N.W. or N.W. The 

 angles of inclination are generally high, though here and there they 

 fall as low as 40° or even less. 



No one visiting this section for the first time would suspect that 

 one half of it is only a repetition of the other half; but when the 



* Quart. Journ. Geol. Soc. vol. xxxi. p. 168. 



