288 A. GEIKIE ON THE SUPPOSED 



In the second place, Dr. Hicks has stated more than once that the 

 Cambrian conglomerates are largely made np of the underlying 

 " Pre-Cambrian " rocks*. As the result of a most careful examina- 

 tion of the conglomerate belt along both sides of the fold, I feel 

 myself warranted in stating confidently that it contains not a single 

 pebble of the characteristic granite of the St. David's ridge. The 

 actual composition of the conglomerates will be best understood 

 from the percentages taken by Mr. Peach on the west side, and by 

 myself on the east side of the fold. 



Percentage of Stones in the Cambrian Conglomerates. 



West side of Isocline. East side of Isocline. 



Quartette (often red) 70 Quartzite (generally red) 59 



Quartz 25 Quartz 35 



Diabase 4 Diabase 4 



Greywacke 1 Greywacke 1 



Eed Jasper 1 



100 100 



Thus, of the component pebbles not less than about ninety-five 

 per cent, are of quartzite or quartz. There are certainly no rocks 

 visible in the district whence these pebbles could have been derived. 

 On the other hand, the quartzite resembles the familiar material 

 that forms so conspicuous a feature in the conglomerates and pebble- 

 beds of all geological ages in this country, and the original source 

 of which it is so difficult to fix. The only fragments of un- 

 doubtedly local derivation are the few pieces of diabase. These came 

 from some of the lavas in the volcanic series below. But such 

 pebbles would naturally continue to be washed off any volcanic islets 

 still rising above the water, until the last mass of lava or consoli- 

 dated tuff had disappeared. Their presence lends no support what- 

 ever to the idea of unconformability. It is well known that similar 



the S.W. of St. David's, is only three quarters of a mile broad. The visible 

 portion of them cannot be thicker than 2000 feet, but is almost certainly 

 less than that. The bottom, however, is not reached, so that they may attain a 

 considerably greater thickness than can be actually seen. 



* His descriptions of the composition of the conglomerate have varied con- 

 siderably from year to year. In 1871, in conjunction wtth Professor Harkness, 

 he described it as " composed chiefly of well-rounded masses of quartz imbedded 

 in a purple matrix," which is a sufficiently accurate diagnosis. In 1877 (Quart. 

 Jouru. Geol. Soc. vol. xxxiii. p. 238) he asserted that "the true Cambrian conglome- 

 rates undoubtedly contain masses of the underlying rocks in their altered state," 

 the conglomerates themselves being unaltered. In 1878 (Q. J. G. S. vol. xxxiv. 

 p. 162) he stated that they are lai-gely made up of the " Pebidian" rocks. And 

 again, in 1881, "the conglomerates at the base [of the Cambrian rocks] are very 

 largely made up of rolled pebbles and rounded fragments identical with the 

 [Pebidian] rocks below " (Proc. Geol. Assoc, vol. vii. p. 63). As there is a singular 

 uniformity in the character of this conglomerate throughout the district, the 

 variation in these lithological descriptions is remarkable. The first notice of 

 this rock appears to have been that by De la Beche, in the paper already cited, 

 where he speaks of it as a " quartzose conglomerate resembling one of Old Red 

 Sandstone." (Trans. Geol. Soc. ser. 2, vol. ii. p. 9.) 



