How TO WORK IN THE ARCH^AN ROCKS. 31 



Archaean groups in Britain are generally brought against the Lower 

 PalfBozoic Kecks, and against each other by faults. In Anglesey and 

 in Ireland, the ground occupied by the Archooans is almost literally 

 a pavement of fragments. The difficulty thus arising may sometimes 

 be surmounted by the following method. An actual example will 

 make the matter clearer. In central Anglesey, there is a broad band 

 of granitoid rock passing down into green schist, but as the area is 

 margined by faults, the succession cannot be traced down lower ; but 

 two miles to the east, we again come upon the green schist, and by 

 foUowiug the section to the west, we find the schist is underlaid by a 

 succession of gueissic rocks. The green schist thus connects the two 

 areas, and enables us to construct a complete succession. 



The test by included fnuiinents is often of great service in these old 

 rocks. Three examples of its value are here given. The plum- 

 coloured conglomerates of the Longmynd (Lower Cambrian), in 

 Shropshire, are largely made up of a purple felstone, which is 

 common in the Wrekin volcanic series, which is thus proved to be 

 Pre-Cambrian. The Wrekin group itself contains conglomerates 

 whose pebbles are varieties of metamorphic rock which have been 

 derived from a series of which Primrose Hill, near the Wrekin, is a 

 denuded fragment. The existence of two Archsean groups in Shropshire, 

 a volcanic and a metamorphic, is thus proved. The third example is 

 in Anglesey. Conglomerates, proved by their fossils to be Cambrian 

 (Tremadoc), contain pebbles of granitoid rock and schist, together 

 with rounded fragments of green and purple slate. It is clear that 

 these conglomerates have been formed from the denudation of the 

 two other formations which occur in the vicinity, a Gueissic and a 

 Slaty group, both of which are thus proved to be Pre-Cambrian or 

 Archaean. 



But the test by included fragments must be used with caution. 

 In volcanic fomiations there may be contemporaneous denudation, 

 and a conglomerate may be derived from a lower part of the same 

 series. Such conglomerates, with pebbles of purple felstone, occur 

 in the Wrekin series, and their included fragments are of no 

 classificatory value. 



The mineral composition of rocks, often an important test even in 

 fossiliferous deposits, as the chalk or the Zech stein, becomes of 

 supreme value amongst the Archsean groups. Thus the green schist 

 of Anglesey, as previously shown, becomes a connecting link between 

 areas separated by faults, and is as readily recognised in any part of 

 the island as if it contained fossils. Thus also the slaty series of 

 Anglesey is inferred to be Pebidian, by its close mineral resemblance 

 to the typical Pebidian of St. David's. Indeed, this test is our chief 

 guide amidst the complexities of these old rocks, and its use has led 

 to some of our most interesting discoveries. Due caution must, 

 however, be exercised in its application. 



This kind of evidence decreases in value as the formations com- 

 pared increase in distance. There are, for example, volcanic rocks 



