9» PEOCEEDINGS OP THE OEOIGGICAL SOCIETT. 



I presume it "will not in the present day be maintained that the 

 metamorphism of rocks over areas of any but very moderate extent is 

 due to the intrusion of veins and erupted masses. The insufficiency 

 of such agency becomes the more obvious when we consider the 

 slight effects produced by even tolerably extensive outbursts, such 

 as the granite of Dartmoor, &c., and in the case of the Malverns 

 there is an absence of any local cause whatever. The more probable 

 explanation in the case of these larger areas is, that they were 

 faulted down, or otherwise depressed, to within the influence of the 

 earth's internal heat, and this is the more likely as they belong to an 

 epoch when the crust is believed to have been thinner. 



Such general and extensive metamorphism, coupled with the 

 events which it has been shown must have subsequently occurred 

 before the laying down of the Hollybush Sandstone, carries the age 

 of these rocks back to a very early period. The physical character 

 of the rocks, and the total absence of conglomeratic beds and quartz- 

 ites, throughout a thickness of many thousand feet, is not without 

 significance when we consider the frequent occurrence of conglome- 

 rates and grits in the Cambrian system ; and in the large proportion 

 of diorite and other hornblendic rocks, and of gneissoid granite 

 which they contain, and in the abundant intermixture of erupted 

 traps, they bear a far closer resemblance to the rocks of Laurentian 

 or Azoic age *. 



The absence of the Lower Cambrian rocks from beneath the 

 Hollybush Sandstone does not materially affect the question ; for 

 the Primordial zone is in direct contact with Laurentian rocks in 

 other parts of the world, e.g. Labrador, Lower Canada, the eastern 

 part of Upper Canada, Scandinavia, Bohemia t, &c. ; and, as already 

 pointed out by Prof. Dana J, there is evidence that, even as early as 

 Azoic times, the surface of the earth was mapped out into an Eastern 

 and a Western continent ; for the Laurentian rocks of North America 

 have never been entirely submerged, and Lower Silurian strata are 

 seen resting horizontally on their upturned edges §, showing that 

 little subsequent disturbance has taken place. So in Northern and 

 Central Europe, rocks of similar age form the general basis of all 

 the other rocks, whether exposed by denudation or upthrust, or from 

 not having been ever entirely covered up, although there are indica- 



* Wlierever rocks of known Cambrian age have been seen metamorphosed, 

 they have been altered into mieaceovis gneiss, or into mica, talc, or ehlorite-scliists, 

 quartzite, &c. These Malvern rocks, on the other hand, are rich in hornblende, 

 a mineral widely distributed throughout the Pre-Cambrian rocks ; and although 

 free quartz is not absent, they contain a large proportion of basic minerals, such 

 as felspar, with a low proportion of silica, hornblende, ferro-almninous mica, 

 epidote, &c., a pecidiarity pointed out by Dr. Sterry Hunt as characterizing 

 the Lam-entian rocks of Canada. This last-named protosilicate is met with both 

 forming veins and as an element of rock-masses, and is noticed by Horner (op. 

 cit. pp. 292 & 293) as also occui-ring in the metamorphic rocks of Cumberland, 

 the Hebrides, and the Channel Islands, all of which belong to the same Pre- 

 Cambrian age as those of the Malverns. 



t Barrande, Bull. Soc. Geol. de France, 2"^ s^rie, vol. x. p. 405. 



% Manual of Geology, p. 732. 



§ Logan, as quoted by Dana, op. cit. p. 142. 



