Dr. C. Callaicay — On the Archcean Roclis, 353 



end of the Wrekin ridge is a conical elevation, called Primrose Hill, 

 composed of metamorphic strata, a hrick-red granitoid rock, com- 

 posed mainly of red felspar and quartz, interbedded with horn- 

 blendic gneiss and granulite, being the predominant type. These 

 beds dip to the north-east, and seem to pass unconformably beneath 

 the volcanic series which dips to the north ; but it is most probable 

 that the junction is a fault; and if so, we have no evidence at this 

 spot of the relations of the two formations. But at Charlton Hill, 

 about two miles to the west, the most satisfactory proof is to be 

 found. Striking east and west through the middle of the hill, and 

 clearly interbedded with tuffs of the volcanic series, is a most 

 interesting conglomerate, containing pebbles of nearly twenty dif- 

 ferent varieties of rocks, chiefly metamorphic. Amongst these 

 are all the prevailing types found in Primrose Hill, and it is im- 

 possible to avoid the conclusion that the conglomerate was derived 

 from a metamorphic group of which Primrose Hill is a denuded 

 fragment. Thus the existence of a second Archsean system in 

 Shropshire is proved. As the Primrose Hill beds are probably the 

 equivalents of the Malvern gneiss, the Shropshire Archgean volcanic 

 series is brought into relation with that group. 



Another interesting example occurs in Anglesey. The con- 

 glomerates which lie low down in the Cambrian rocks of the island, 

 and which, by their fossils, are proved to be of Tremadoc age, are 

 mainly derived from two older formations, the gneissic and the 

 slaty series. West of Llanfaelog, these conglomerates are associated 

 with fossiliferous grits, and contain pebbles of granitoidite, and of 

 green and purple slate, with other types common in the two altered 

 series, which are thus shown to be Archaean. Fi'om such examples 

 as these the testimony of included fragments is shown to be of great 

 value in certain cases. 



But this test must be used with caution. In volcanic formations, 

 conglomerates may be derived from lower beds of the same series. 

 If, for example, a stream of lava from Mount .^tna were to reach 

 the sea and become consolidated, the waves might undermine the 

 newly-formed rock, break up the fallen fragments, and round them 

 into pebbles. Thus a shingle-bed would accumulate, and this might, 

 by the addition of cementing matter, become a conglomei-ate. 

 Pebbles of quartz or any other rocks which were in the vicinity 

 might be mixed up with the lava shingle. If the volcano grew, this 

 conglomerate in process of time would probably be covered in by 

 new lava-flows, or by beds of ashes ejected from the crater. Such 

 conglomerates occur in the Wrekin volcanic series. The pebbles 

 are usually a purplish felsite, but a small proportion of them are of 

 quartz. These beds are obviously produced by contemporaneous 

 denudation, and the included fragments of lava are of no value in 

 classification. 



{To be concluded in our next Number,) 



DECADE II. — VOL. VIII. — NO. VIII. 23 



