4Q ARCHAEAN. 



This granite he maintains, instead of being a ridge of Pre-Cambrian 

 metamorphic rock, is really a boss of eruptive granite, later in date 

 than the Cambrian strata through which it has been intruded. 



Turning to the Arvonian rocks of Dr. Hicks, Dr. Geikie remarks 

 that " Instead of finding evidence that these rocks lie with a dis- 

 cordant strike unconformably against the so-called ' Dimetian' 

 below, and are covered unconformably by Cambrian or 'Pebidian' 

 beds above, Mr. Peach and I discovered that Dr. Hicks had really 

 created a separate stratigraphical ' group ' out of the zone of 

 quartz-porphyry bosses and dykes with the accompanying indurated 

 sedimentary rocks that surround the central core of granite." 

 Passing on to the Pebidian rocks, Dr. Geikie agreed with Dr. 

 Hicks that the group marked on the Geological Survey Map as 

 altered Cambrian is almost entirely of volcanic origin, as originally 

 recognized by Sir A. C. Ramsay, and formed principally of tuffs 

 with bands of olivine-diabase and occasional intrusive masses of 

 quartz-porphyry. Dr. Geikie, however, sought in vain for proofs 

 of any unconformity between these rocks and the so-called 

 Arvonian — in fact the volcanic groups (Pebidian) can be seen in 

 many places graduating insensibly into the altered shales which 

 form a great part of the Arvonian. Indeed, the latter group (says 

 Dr. Geikie) consists of portions of the volcanic breccias and tuffs 

 (Pebidian) where these are invaded by quartz-porphyry. Further- 

 more, Dr. Geikie asserts that the volcanic (Pebidian) group and 

 the overlying Cambrian conglomerate, sandstone, and shales are 

 perfectly conformable — for volcanic beds are even interstratified 

 with the conglomerate and appear above it. Moreover, " On the 

 shore of Ramsey Sound, at the headland of Castell, the red shales 

 and sandstones with Lhigulella prmcEva, which lie not far above 

 the conglomerate, are banded with thin seams of sandy tuff, some 

 of the shales being also full of diffused tuffaceous material, as if 

 from slight discharges of fine volcanic dust during the last stages 

 of eruption in the district."^ (See also under Cambrian, p. 56.) 



1 hese differences in the interpretation of this ancient ' Geological 

 Record' are serious, but perhaps not quite so serious as they at 

 first appear. It is admitted that the rocks termed Pebidian under- 

 lie the oldest fossiliferous Cambrian strata, and rest on the rocks 

 termed Arvonian. The relative position and the origin of the 

 Dimetian granitoid rock are the main points in question. It may 

 be mentioned, however, that in other areas, where Archjean rocks 

 have been identified, two main divisions are recognized, one of 

 coarsely crystalline rocks (Dimetian), and the other of eruptive 

 rocks (Pebidian)." 



Mr. W. H. Hudleston, in speaking of these old rocks, has 

 observed that in the more crystalline lower series, as developed 

 near Caernarvon and St. Davids, we see the hypogene conditions 

 of a great mass of rocks, whilst the beds between these crystalline 

 masses and the great Cambrian conglomerate represent a more or 



^ Q. J. xxxix. 261. See also J. F. Blake, Q. J. xl. 294. 

 2 C. Callaway, G. Mag. 1885, p. 260. 



