162 H. HICKS ON THE DIMETIAN AND 



invariably show massive conglomerates of well-rolled pebbles in- 

 dicating the prevalence of beach- conditions over those areas, either as 

 shore-lines to islands in shallow water, or coast-lines of an uneven 

 land which was once more becoming depressed below an encroaching 

 sea. The pebbles, as already shown, also almost invariably for the 

 most part consist of fragments of the rocks upon which they lie ; 

 and when examined they tell distinctly that tho underlying Pebidian 

 rocks, after they were elevated, when these fragments were broken 

 off, and before they were again depressed, were not only consolidated, 

 but also nearly as highly metamorphosed as they are found to be at 

 present. 



The changes referred to may be briefly summarized as under : — 



Dimetian. 



1. Depression of pre-Dimetian land. 



2. Deposition of the sediments, accompanied by some volcanic 

 action. 



3. Elevation, accompanied by consolidation. 



4. Land areas with volcanoes. 



Pebidian. 



5. Depression of the land. 



6. Deposition of loose materials washed off from the Dimetian 

 land with gradual submergence. 



7. Heaping-up of materials chiefly derived from submarine 

 volcanoes, accompanied with moderately rapid depression. 



8. Re-elevation of the areas depressed, probably to a considerable 

 height, with consolidation of the rocks. This change, which pro- 

 duced the immediately pre-Cambrian land, was undoubtedly ac- 

 companied in places by considerable fractures in the previously 

 consolidated Dimetian rocks, as the Pebidian beds are seen to lie 

 across the edges of these rocks and in positions which I think they 

 could not have assumed after perfect consolidation in themselves. 



The great faults, however, which are met with in these areas 

 were undoubtedly produced at a much later period, probably about 

 the close of the Palaeozoic, and were doubtless due in a measure to 

 the indurated state at the time of these two early formations, and 

 to their readiness therefore to fracture rather than to bend in the 

 great movements which took place in the crust of the earth at the 

 close of Palaeozoic time, when the Cambrian and succeeding sedi- 

 ments were readily bent into folds. 



Note (March 18, 1878). — There can be no doubt in the mind of 

 any one who carefully examines the various sections of these rocks 

 at St. David's, that there must be here at least the representatives 

 of two very distinct formations, evidently deposited under different 

 physical conditions, and hence unlike lithologically. That they are 

 unconformable to one another also seems beyond doubt ; for by no 

 other means could they, as they do at all points, show a persistently 



