THE ARCHAEAN ROCKS 53 



surface farther north, one extending from Uanllytin on the 

 to Penrliyn on the north, a distance of about 15 miles, with a 

 maximum width of 2 miles (see map, Fig. 1! This tract conflict* 

 of a rhyolitic felsite succeeded by breccias and schista 



A second tract extends from Carnarvon to Bangor, a distance of 

 about 10 miles, with a width of over a mile in the central part 

 This latter has been well described by Professors Bonney and 

 Hughes, 19 who have shown that there are three seta of rocks in 

 the ridge, (1) a granitoid rock at the southern end (Twt HiU), (2) 

 a reddish - purple quartz -felaite, (3) a series of felspathic grit*, 

 slates, and agglomerates (the "altered Cambrian" of the Geological 

 Survey). These last are overlain by other conglomerates of Lower 

 Cambrian age. With respect to the relations of these rocks the 

 following facts appear to be well established : 



(1) That the quartz -felsite was a rhyolite, and probably a 



rhyolitic lava-flow. 



(2) That between this rhyolite and the basal Cambrian con- 



glomerate there is a series of volcanic and sedimentary 

 deposits which are not less than 3000 feet thick. 



(3) That the Cambrian conglomerate contains many fragments 



of the quartz-felsite and some of granitoid rock (consisting 

 of quartz and felspar) like that of Twt Hill. 



This set of pre-Cambrian rocks has been called the Bangor 

 Series, and it should be noted that this series differs from that found 

 in Anglesey and the Lleyn in several particulars. The volcanic 

 rock is acid not basic, all the rocks are less altered and less flexured, 

 and they directly underlie the base of the Cambrian System. 



Pembrokeshire. The oldest rocks in South Wales crop out 

 in the neighbourhood of St David's (see map, Fig. 16). A pre- 

 Cambrian age was first claimed for them by Dr. H. Hicks in 1871, 

 and the results of his examination of the district were published in 

 several papers. 20 His conclusions may be thus summarised : that 

 the base of the Cambrian System is marked by a conglomerate 

 containing pebbles of the underlying rocks ; that this conglomerate 

 rests on a series of schistose volcanic tuffs and breccias, under which 

 is another set of quartx-felsites and halleflintas. Below these again 

 comes a granitoid rock which he regarded as a still older mass of 

 metamorphic rock. To this last he gave the name of IHmttian ; 

 the overlying volcanic series he called Pebidian (see Fig. 6). 



In 1883, however, these conclusions were called in question 1-v 

 Sir A. Geikie, who maintained the correctness of the Geological 

 Survey mapping, holding that the granite is an intrusive maw 

 of igneous rock, and that the volcanic series (Pebidian) is a down- 



