43« 



NATURE 



[September 28, 191 1 



initialled the curious courses of the nvers "I 

 the district, rhe intricate channels and harbours of the 

 Hampshire coasi are the product of this long denudation 

 combined with oscillations of land-level. The Hampshire 

 and Sussex coasts yield particularly clear evidence of the 

 curious alternations oi climate which characterised the 

 Pleistocene period. The ice-sheet did not extend so far 

 south, but there is abundant evidence of two cold periods 

 separated by a milder stage. Man must have seen many 

 of the later of these changes. He certainly hunted the 

 reindeer on Salisbury Plain, and probably hunted the seal 

 and walrus amid the pack-ice of the Portsmouth coast. 



Prof. S. H. Reynolds gave a description of further work 

 on the Silurian rocks of the Eastern Mendips, in which 

 the results of opening a series of fresh trenches were 

 detailed. The fossils obtained showed that the sandy mud- 

 stones to the south-east of the Moon's Hill Quarry are of 

 Wenlock rather than of Llandovery age. The strike of 

 these Wenlock rocks is completely discordant with that of 

 the underlying Old Red Sandstone, and precludes the 

 possibility of a conformable passage from Silurian to Old 

 Red Sandstone. No trace has been found of a Ludlow 

 fauna. The dip of the Wenlock rocks is such that they 

 clearly overlie the andesite of Moon's Hill. 



Dr. A. R. Dwerryhouse outlined his recent investigations 

 on the glaciation of the north-east of Ireland. He con- 

 cluded that the area including the Antrim basaltic plateau, 

 the Silurian uplands of County Down, and the Mourne 

 Mountains had been completely overriden by the Firth of 

 Clyde glacier, during the retreat of which the drainage 

 of the district was impounded and a number of lakes 

 formed. The overflow channels of these lakes have left 

 " dry gaps," which mark the various stages in the 

 shrinkage of the ice, the earliest of which appear on the 

 flanks of the Mourne Mountains at an elevation of 1200 

 feet. Many interesting examples of the diversion of rivers 

 by morainic material occur, and the changes in the history 

 of the Lough Neagh area were outlined. It has been 

 stated that the Antrim plateau was glaciated by local ice 

 after the retreat of the Firth of Clyde glacier ; but Dr. 

 Dwerrvhouse had failed to find any direct evidence of this, 

 though the valleys of the Sperrin Mountains to the west 

 were occupied by local glaciers. 



The Glacial period and climatic changes in north-i ast 

 Africa was the subject of a paper by Dr. W. F. Hume 

 and Mr. J. I. Craig. They predicted a southerly shift in 

 the system of westerly moist winds of the northern hemi- 

 sphere due to the ice-cap by several degrees, with a 

 decrease in temperature below the normal. These winds, 

 which now barely touch the north coast of Egypt in winter, 

 would then impinge upon the loftiest portions of the Red 

 Sea mountain range. Evidence of this increased rainfall 

 is found in the gravel terraces on their western slopes, 

 the materials of which could only have come from the 

 hills to the north-east. The precipitation was most active 

 where the range was highest. Further evidences of such 

 a westerly current are to be found in the existence of 

 calcareous tufas on the borders of the eastern scarp of 

 Kharga Oasis and elsewhere. The authors also found 

 evidence of changes in the monsoon effects during the 

 Glacial period. It is known from the investigations of 

 the Meteorological Department of India that an increased 

 snowfall in the Himalayas in spring exercises a measurable 

 prejudicial effect on the Indian monsoon of the present 

 day, and the enormously greater ice-covering of the Glacial 

 period would exercise a more powerful inhibition on the 

 monsoon of that period. The more extensive ice-sheel 'if 

 Fast Africa, b; preventing the abnormal heating of the 

 land in summer, would net further in the same direction, 

 and it is extremely probable that the monsoon current 

 partook of the southerly displacement. The general result 

 would he a decreased precipitation over Abyssinia and a 

 much reduced Sobat, Blue Nile, and \tharn, which at the 

 present account for 06 per cent, of the flood proper of 

 the Nile. The study of the alluvial muds of the Lower 

 Nil.' indicates a much smaller rainfall in Abyssinia about 

 14,000 years ago, previous to which the mud-laden waters 

 of the Abvssinian Nile system do not seem to have reached 

 Kgvpt. Mr. Grabham confirmed this conclusion by 

 evidence from the Sudan of an earlier, moister climate 

 further south than now is the case. 



NO. 2187, V0L - 87] 



Friday, September 1, was devoted to a joint meeting, 

 with Section L [Geography), which was opened by a dis- 

 cussion on the former connection of the Lie of Wight with 

 the mainland, the subject being introduced by Mr. 

 Clement Reid, F.R.S. Mr. Reid pointed out that the 

 Solent and Spithead occupy parts of a wide river-valley 

 in which terraces of gravel slope up to 400 feet, though in 

 the centre of the valley they pass actually beneath the 

 present sea-level. These gravels have a very peculiar 

 composition, and the higher terraces are full of Green- 

 sand chert, and contain fragments of Palaeozoic rocks 

 belonging originally to the West Country. The old idea 

 that these cherts came from the central area of the Weald 

 has been found to be untenable, and it has now been found 

 possible to trace these stones to their origin, and so unfold 

 a beautiful example of river development and river destruc- 

 tion. When first earth-movements formed the Tertiary 

 basins of Hants and London, each of these basins was- 

 closed by harder rocks to the west, and was occupied by 

 an eastward-flowing river, the Thames and the Solent. 

 The valley of the Thames seems merely to have deepened, 

 retaining all along approximately its original course. 

 The valley of the Solent, on the other hand, ran for some 

 distance parallel to the sea, and at no great distance from 

 it, the result of which was that the sea finally broke 

 through the narrower ridge of chalk which once ran con- 

 tinuously from the Needles to the Dorset coast, thus- 

 diverting the Frome and all the western rivers from their 

 natural course to the Solent, and isolating the Isle of 

 Wight. This flank attack had still other effects, for the 

 Lower Avon was then only a short river, having a gentle 

 fall of many miles before reaching the sea somewhere near 

 Portsmouth. Subsequently it reached the sea bj 

 steep, direct course, and consequently, as the Lower Avon 

 •flowed over soft Tertiary strata, it lowered its bed so 

 rapidly as to cut back its valley and capture the whole 

 of the drainage of Salisbury Plain, which previously had 

 continued its natural course to Southampton Water. Clear 

 evidence of this diversion and alteration of drain, ij 

 was forthcoming in the high-level gravels of the Vale of 

 Wardour, containing peculiar fossiliferous Purbeck cherts, 

 which went straight across the present Avon Valley and 

 were found on its west side, showing that when the rivers 

 flowed some 300 feet higher they were tributaries 01 

 Southampton Water. Thus the great River Solent had all 

 its headwaters cut off, and was divided into 



river-basins, each with its own outlet. This 

 happened in late Pliocene times. These flank attacks are 

 still going on further west, and if they continue much 

 longer the breach at Lulworth Cove may widen and deepen 

 in the same way, so that with slight submergi 

 so-called Isle of Purbeck may become a true island exactly 

 comparable in its geological structure with the Isle t§ 

 Wight. Though at this early period the Isle □ 

 was cut off from the mainland, it was probably at first 

 only separated bv a small stream and marshes, and was 

 sometimes an island, sometimes part of the mainland, as 

 the sea-level varied. The final isolation took place eil 

 quite a recent period, as the Isle of Wight was probably 

 the ancient Ictis or Vectis of classical writers, to which 

 the ancients traded for tin, and which is described as being 

 cut off at high tide, but connected at low tide by a narrow 

 causeway. The causeway was probably the ledge of Bern- 

 bridge Limestone which swept across what is now the 

 Solent from Yarmouth to Hurst Castle, and was intact 

 ago. It has now been destroyed by the attacks 

 1 ,1, and was apparently impassable at the Roman 



occupation, for the roads then led to a ferry further east 

 and out of the run of the sea. 



Dr. J. W. Evans thought that the submergence which 

 was the immediate cause of the separation of the Isle of 

 Wight was conn. cted with the disappearance of glacial 

 conditions from this country. The presence of accumula- 

 tions of ice appears to cause a local lowering of the 

 earth's crust accompanied by compensatory elevation in 

 adjoining areas which, like the south of England, weta 



i. When the ice passed away these movements 



1 sed in direrlion, as is now- seen in Scandinavia. 

 The period of elevation in our southern roast coinciding 

 with II,.. maximum of the glaciation of the British Isles 



of great precipitation, and it was then that the 



