260 Br. C. Callaicaij — On Comparatue Lithologij. 



I feel myself on firmer ground in Shropshire. The gneiss of 

 which Primrose Hill is a fragment is highly crystalline. It is older 

 by an interval than the Wrekin volcanic series (Uriconian), for it 

 has furnished pebbles to Uriconian conglomerates. The Wrekin 

 group is either volcanic or hypocrystalline, and it is certainly older 

 than the Longmynd series, for the purple conglomerates and sand- 

 stones of the latter are largely derived from it. There is a third 

 Archgean group in Shropshire, the quartz-schists of Rushton, but 

 these I leave on one side. The existence of at least tw^o Archsean 

 systems in the Wrekin area I hold to be absolutely certain. 



In Caernarvonshire also there are tvpo groups, a granitoid mass 

 like the Dimetian, and a volcanic series similar to the Uriconian. 

 Both of these are Archaean, since Harlech conglomerates contain 

 pebbles derived from them. 



Anglesey likewise affords proof of two ancient systems. The 

 older is gneissic and granitoid. It furnishes rounded fragments 

 both to a hypocrystalline series and to the lowest Palaeozoic rocks 

 in the island. The hypocrystalline group supplies pebbles and large 

 angular pieces to conglomerates which are of at least Arenig age, 

 and may be older. 



These facts receive confirmation from other areas where the 

 evidence is less satisfactory. The Malvern chain, consisting of 

 highly crystalline gneiss, is flanked on the east side by altered rocks 

 which closely resemble Pebidian types at St. Davids and in Shrop- 

 shire. In the Midlands, near Nuneaton, volcanic rocks of the 

 Charnwood series are said to underlie quartzites which are certainly 

 not younger than the Cambrian. 



The conclusions thus briefly stated represent the independent 

 observations of several well-known geologists, each differing from 

 the rest in some points, as might be expected from the difficulty of 

 the work, yet agreeing with them in the main facts. As I have 

 worked out some of these results, and personally verified nearly all 

 the rest, I can speak with some confidence, and I do not hesitate to 

 say that the existence in England and Wales of two well-marked 

 groups older than the Cambrian has been clearly proved. The 

 younger of these systems is usually volcanic ; but the centres of 

 activity are, as we should expect from analogy, sporadic. In the 

 Wrekin, massive lava-flows alternate with ash-beds, breccias, and 

 bands of hornstone ; but a few miles to the north-east, in Lilleshall 

 Hill, rhyolites are apparently absent, the ridge consisting of well- 

 bedded ashes, hornstones, and altered shales, together with some 

 masses of volcanic breccia. These strata are precisely such as would 

 be formed by volcanic action at a distance from the centre of 

 eruption. The altered shales and hornstones form a connecting 

 link between the typical volcanic products and the hypometamorphic 

 schists of some other districts. 



In North Wales, the phenomena are similar ; but we can go a step 

 further. The rhj'olites of Llyn Padarn and south of Bangor are 

 obviously not far from volcanic foci. They are oveidain near 

 Bangor by conglomerates and grits, both of which are largely 



