56 CAMBRIAN. 



where the rocks are exhibited.^ In descending order the beds are 

 grouped as follows : — 



/ Upper. — Grey flags, 150 feet, with Obolella sagittalis, Paradoxides 

 I aurora, Conocoryphe biifo, 



2. Solva 1 Aliddle. — Coloured gritty and flaggy rocks, 1 500 feet, with Para- 

 Beds. \ doxides Solveusis, Aguostns Cainbrensis, Eophyton. 



f Lo7ver. — Yellowish flags and grits, 150 feet, with Paradoxides 

 ^ Harhtessii, Plutonia Sedgwickii, Eophyton. 



f Upper, — Massive purple sandstones, 1000 feet, with Annelides. 

 _ ^ .\ Middle. — Red shales, 50 feet, with Li>igiilella prirncBva, Discina 

 ' tT ^ ^^ \ Caerfaiensis, Lepcrditia Camhrensis. 



^ ^' i Lower. — Green flaggy sandstones and conglomerates, 520 feet, 

 1 with Fucoids and Annelides. 



The Annelides are represented by Armicolites and Scolitcs. Not- 

 withstanding the fossils mentioned, the rocks, as a rule, are very- 

 barren. 



Dr. Hicks considers that the older (Pre-Cambrian) rocks form a 

 distinct group, and that the Cambrian conglomerates overlap them 

 irregularly. These conglomerates, in his opinion, are almost 

 entirely made up of pebbles derived from the underlying rocks. ^ 

 The Lower Cambrian rocks are well shown in the Caerbwdy 

 Valley, and along the coast towards Nun's Chapel. In the 

 Caerbwdy Valley, the thickness of the more conglomeratic portion 

 of the basal beds is about 60 feet, and some of the pebbles attain a 

 foot in diameter.^ 



Dr. A. Geikie has thus divided the Lower Cambrian (Harlech) 

 rocks of St. Davids, exposed on the north side of St. Bride's Bay in 

 Pembrokeshire :^ — 



Purple and green grits and shales s g^j^^ ^^^ 



Purple grits _ Caerfai 



Red shales (with Lmgidella primceva). ) ' ' r 



r- . , , , ^ ^ ' I groups 01 



Greenish shales 1 -p? w" V 



Quartzite conglomerate ' 



Contemporaneous f Porthlisky schists \ Pebidian of 



volcanic group. \ Agglomerate, tuff; and diabase ) Dr. Hicks. 



In this grouping, as before noted. Dr. Geikie's conclusions are at 

 variance with those of Dr. Hicks. (See p. 39.) The conglomerate 

 has been taken as marking the base of the Cambrian system, but 

 Dr. Geikie believes that no abrupt break can be traced between the 

 volcanic group and the conglomerate. The latter forms a band 

 composed of rolled pebbles of quartz and quartzite, whose average 

 size is probably less than that of a walnut, though occasionally as 

 large as a man's head. The band varies in thickness from a few 

 feet to upwards of one hundred feet. The conglomerate appears 

 to mark the decline of the submarine volcanic activity. Dr. Geikie 



^ Q. J. xxvii. 396. See also Proc. Geol. Assoc, vii. 282 ; Pop. Science 

 Review, 1881 ; and C. Lapworth, G. Mag. 1881, p. 321. 



^ Q. J. xxxiii. 230. 3 Q. J. xl. 520. 



* Q.J. xxxix. 268. See also Hicks, Q.J. xl. 526. 



