616 REPOET— 1886. 



continental land resemUed the larger streams of Europe they would suffice for the 

 transport of the materials with which we have dealt, especially if aided hy coast 

 currents after the debris had reached the sea. 



(8) If boulders occur in a matrix consisting^ of fine mud, or mainly of organic 

 material, they must (unless they are volcanic bombs) have floated thither either 

 attached to large seaweeds or entangled in the roots of trees, or supported by ice. 

 If they are rather numerous and a foot or more in diameter, in a marine deposit, 

 the last is the most probable mode of transport. A cubic yard of ice will more 

 than sufiice to float a cubic foot of average rock. 



Conclxision. 



The facts already mentioned, regarded in the light of the above principles, 

 justify, in my opinion, the following inferences as to the past physical geography 

 of our country. At the commencement of the Cambrian period great masses of 

 Archaean rock, granites, gneisses, aind schists must have existed, not only on the 

 western side of Britain, but also over a considerable tract of land now covered by 

 the sea. Detritus from this continent became an important constituent in the Cam- 

 brian rocks, and in many cases, as at St. David's, in Anglesey, Carnarvonshire, 

 &c., the .shore-line must have been very near at hand. "With the Cambrian period 

 commences a long-continued subsidence, so that its basement beds at different 

 places are very probably not all of quite the same age. The land surface was 

 from the first irregular, and it is very probable that waves of the sea were fretting 

 away some parts, while rain and river, heat and cold, were still sculpturing others. 

 But among the materials of the ancient land were not only granitoid rocks, 

 gneisses, and schists, but also newer strata more distinctly of clastic origin, 

 memorials of past denudation — quartzites and grits, phyllites and slates, not to 

 mention others — and these, by their intimate structure, sometimes indicate that 

 gi'eat earth-movements must have already occurred.' In many localities, perhaps 

 as a result of these disturbances, there occurred, towards the conclusion of the 

 Archaean period, great volcanic outbursts — by which, no doubt, numerous cones 

 were built up, and many of the materials of the so-called Pebidian Group 

 were supplied. It is, I think, at present hardly safe to attempt to trace the 

 exact laud boundaries of the Cambrian ocean, but the enormous masses of 

 Archaean material which are entombed in the earlier Palaeozoic strata of Wales 

 and of North-west Scotland can, I think, only be explained by the proximity 

 of a great continental land — an extension of the present Scandinavian Peninsula 

 — which not improbably had a general slope towards the south-east, the main 

 watershed of which may have lain some distance to the west of the Outer 

 Hebrides.'^ But even over the more central parts of Britain there cannot have 

 been deep or open ocean. We are constantly coming upon the traces of pre- 

 Cambrian and early Cambrian land ; some of our Mid-England Archtean masses, 

 like the Malverns, appear to have risen above the water, and to have undergone 

 denudation after the great earth-movements which ushered in the Silurian period. 

 Prior to this, after a time of repose in the Cambrian, at more than one epoch, and 

 in more than one place, there were great volcanic outbursts, which appear to have 

 studded the sea with volcanic islands, and to have added to the heterogeneous 

 materials from which the .sediments were now formed. It is evident that in 

 Silurian times the coast-line had extended southward and eastward. The coarse 

 deposits of this age, in Wales, the Lake district, and Southern Scotland, compared 

 with the finer mudstones and limestones of the Welsh border and of England, seem 

 fully to bear out this assumption, which is in accordance with a well-known law 

 of mountain-making. The Old Pied Sandstone of Scotland and of Wales indicates 



' It is evident, for instance, that the north-west strike, and other effects of 

 folding, had been produced in the Hebridean series of N.W. Scotland before the 

 Torridon sandstone was deposited. • 



^ Possibly the comparatively rapid deepening of the Atlantic beyond the 100- 

 fathom line may have some relation to the western outUne of this primteval 

 Atlantis. 



