294 A. GEIEXE ON THE SUPPOSED 



fords the most complete confirmation of the results obtained by us 

 in our observations in the field. 



1. Order op Succession oe the Bocks. 



The extent of the coast-sections, and the repetition of the strata 

 on two sides of an axis of plication, combine to furnish the most 

 satisfactory data for compiling a vertical table of the rocks. But 

 the base of the whole series is not seen. As the line of fold seems 

 to be dying out towards the south-west, it probably for a time brings 

 up progressively lower beds as it is traced inland in the opposite 

 direction, until it flattens out and disappears. The oldest rocks 

 visible to the west and south-west of St. David's belong to the inter- 

 esting volcanic group referred to in preceding pages. From these 

 a continuous passage can be traced upwards into the purple, grey, 

 and green sandstones and shales from which Dr. Hicks has obtained 

 so abundant a Lower Cambrian fauna. The following groups of strata, 

 from their easily recognizable lithological characters, may be taken 

 as a convenient series for enabling the observer to trace out the 

 general geological structure of the district. 



Groups of Lower Cambrian {Harlech) strata at St. David's, 

 in descending order. 



4. Purple and greenish grits, sandstones, and shales. 



3. Green and red shales and sandstones, tuffaceous in parts. 



2. Quartz conglomerate. 



1. Volcanic group (tuffs, schists, lavas, &c). 



1. Volcanic Group. 



The rocks comprised in this group present so many points of 

 interest that they deserve, and would well reward, a much more de- 

 tailed study than I have had an opportunity of giving to them. 

 They are the oldest visible portions of the Cambrian system in 

 the St. David's district. They consist almost wholly of volcanic 

 materials, consolidated tuffs and breccias with contemporaneously 

 erupted and subsequently intruded massive rocks. They are exposed 

 in so many sections, and so continuously, both along the shore-cliffs 

 and inland, that their succession and structure could be worked 

 out with little difficulty. If not the oldest group of truly volcanic 

 masses in Western Europe, they (and their equivalents in other 

 parts of Wales) are, as I have said, at least the oldest of which the 

 precise stratigraphical place in the geological record is known. 

 They thus furnish important evidence to the student of the history of 

 volcanic action. 



In my examination of these rocks in the field, I was especially 

 struck by their general resemblance to volcanic masses of later 

 Palaeozoic date. Many of the lavas and tuff's are in outward charac- 

 ters quite undistinguishable from those of the Lower Old Eed Sand- 

 stone and Carboniferous systems of Central. Scotland. So many points 



