Dr. C. Callaway — On Comparative Lithology. 261 



derived from rhyolitic material. At the city of Bangor, a little 

 further from the lava-masses, we come to agglomerates and altered 

 slates, the latter made up of " volcanic dust." In Anglesey, we are 

 at a still greater distance from the centre of eruption. Lavas are 

 seemingly wanting, but almost every other Pebidian type known to 

 me in South Wales and Shropshire occurs. The most abundant 

 varieties are felspathic breccias, hornstones, and fine-grained altered 

 shales. These rocks are older by an interval than (at least) the 

 Arenig period, and they are utterly unlike any part of the Cambrian 

 group, so that their Pre-Cambrian age is almost certain, even if we 

 disregard their close resemblance to the Pebidian. Associated with 

 these Pebidian types, in such a way as to prove that they are parts 

 of the same system, we find pale-green slates, like those of Charn- 

 wood, grits, conglomerates, quartzites, dolomites, and chloritic 

 schists. The grits and the matrix of the conglomerates often display 

 alteration, the slates are frequently hypocrystalline, and the schists are 

 sometimes almost as highly crystalline as those of Holyhead. This 

 region has been affected by powerful lateral pressure, giving rise to 

 complex contortion and thrust-planes ; but whether or not the earth- 

 thrust has produced the metamorphism does not concern my present 

 inquiry. I will merely observe that the ci-ystallization existed in Pre- 

 Cambrian times ; and that the Ordovician rocks have been intensely 

 crumpled and repeatedly sliced by thrust-planes, yet they have under- 

 gone no metamorphism beyond the cleavage stage. 



Our studies seem then to prove that in South Britain crystalline 

 schists in regional masses occur only in Archaean groups, and that of 

 the two well-established systems, the younger exhibits but partial 

 crystallization. These facts appear to shape themselves into a law, 

 empirical at present, but of sufficient authority to raise a strong pre- 

 sumption in favour of its application to a wider area. 



If then we cross St. George's Channel, and find a Pre-Cambrian 

 series with mineral and petrological characters similar to the 

 Pebidian, displaying also a hypocrystalline structure, varying in like 

 manner between slight alteration and fairly distinct schistosity, are we 

 rash in assigning such a group to the Pebidian system ? If this 

 inference be resisted, if belief is refused until we can trace Pebidian 

 rocks under the sea from Anglesey to Ireland inch by inch, I dare 

 to think that such scepticism is irrational and destructive of scientific 

 progress. 



I have already, in this Magazine, November, 1881, p. 494, given 

 reasons for believing that in the south-east of Ireland there are two 

 Archeean groups, probably corresponding to the crystalline and hypo- 

 crystalline formations of Anglesey. An important fact, which I 

 wish to recall, is that, in the region south of Wexford, the direct 

 proof of Archaean age is very strong. The masses of gneissic and 

 hypocrystalline rock lie side by side with areas of Cambrian and 

 Ordovician strata ; yet I could nowhere find evidence of a passage 

 between any two of the groups. The Paleeozoic slates and shales 

 displayed no alteration, but abutted against the schistose rocks with 

 sharp lines of junction. 



