252 PROF. C. LLOYD MORGAN ON THE TEBIDIAN 



suppositions. I am therefore forced to conclude that Pebidian and 

 Cambrian here interdigitate. 



f. The Materials of the Conglomerate. — " The pebbles '" [of the 

 conglomerate] says Dr. Hicks, " almost invariably for the most part 

 consist of the rocks upon which they lie, and when examined they 

 tell distinctly that the underlying Pebidian rocks, after the}' were 

 elevated, when these fragments were broken oiS, and before they 

 were again depressed, were not only consolidated but nearly as 

 highly metamorphosed as they are found to be at present." 

 (Q. J. G. S. xxxiv. p. 162.) On the other hand, Dr. Geikie ?ays (Z. c. 

 p. 288), " Of the component pebbles not less than about 95 per cent, 

 are of quartzite or quartz." And, as the results of the examination 

 of microscopic slides, Mr. T. Davies says (Q. J. G. S. xl. p. 550), 

 " The contents of the Cambrian conglomerates of St. David's, found 

 in the sections examined are : — quartz, both dirty and clear, small 

 pebbles consisting of dirty quartz with two felspars, individual felspars 

 both orthoclase and plagioclase, felsite, quartz felsite, quartz-schists, 

 quartzite, basic rocks, porcellanite, and mica much altered." 



Now there can be no question that a preponderating number, I 

 may say a very largely preponderating number, of the pebbles are 

 composed of materials from a foreign source, and for the most part 

 of quartz and quartzite. Put there are, especially in some localities, 

 other included fragments. These are particularly conspicuous at the 

 Arch, near Maen Bachau, south of Castell. A goodly number of these 

 may be fairly matched with undoubted Pebidian. This at first im- 

 pressed me strongly in favour of Dr. Hicks's contention. Put 

 when I came to study the Pebidian beds more carefully and in 

 greater detail, I found the fragments imbedded in the tuffs and 

 agglomerates so numerous, in some cases so large, and so closely 

 resembling the fragments included in the conglomerate that it 

 seemed to me that the evidence afforded by them was by no means 

 so clear as I had at first supposed. 



These Pebidian fragments, if such they be, enclosed in the cou- 

 glomerate on Eamsey Sound, are remarkable in that they are for 

 the most part much softer than the more numerous quartzite- and 

 quartz-pebbles, and that they are less well rounded. In some cases 

 they form long flat subangular slabs, in others somewhat lenticular 

 masses ending in fine points ; occasionally they seem to be indented 

 by the harder and rounder pebbles. Clearly they have not been 

 brought from a great distance. Clearly they have not been rounded 

 by long-continued wave-action. 



During a second visit to St. David's for pur^wses of verification, 

 I paid especial attention to the questio]i of included fragments, 

 both in the Pebidian and in the Cambrian conglomerate. In the 

 former there are a considerable number of fragments, from what- 

 ever source they may have ])een derived, of a horny porccllaiiitic 

 rock. Most of these are small, though fragments from half an inch 

 to an inch in length are not unfrequent, Pragments of porcellanitic 

 rock in the conglomerate may therefore be from this source. I 

 was again struck with the number and variety of the fragments of 



