HOW TO WORK IN THE ARCH.EAN ROCKS. 213 



their summits thrown over to the north-west. The contorted schists 

 of Anglesey display the same phenomena. The contortions have been 

 disentangled bythe discovery by the writer of a grey gneiss underlying 

 the prevailing green schist, and the latter is seen to he in sharp 

 synclinal folds between anticlines of the gneiss. 



The superposition test is also complicated by faulting. The 

 Archaean groups in Britain are generally brought against the Lower 

 Palaeozoic Rocks, and against each other by faults. In Anglesey and 

 in Ireland, the ground occupied by the Archaeans 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 

 following the section to the west, we find the schist is underlain by a 

 succession of gneissic rocks. The green schist thus connects the 

 two areas, and enables us to construct a complete succession. 



The test by included fragments 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 Archaean groups in Shropshire, a 

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

 in Anglesey. Conglomerates, proved by their fossils to be Cam- 

 brian (Tremadoc), contain pebbles of organitoid 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 iu the vicinity, a Gneissic 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 formations 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 classifi- 

 catory value. 



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

 fossiliferous deposits, as the chalk or the Zechstein. becomes of supreme 

 value amongst the Archaean groups. Thus the green schist of Angle- 

 sey, 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 



