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SCIENCE 



[N. S. Vol. XXXII. No. 820 



of the other, seeing that the origin of each 

 is equally dubious. Given a sufficient 

 "head" of ice in northern regions, it might 

 be possible to transfer the remains of or- 

 ganisms from the bed of the Irish Sea to 

 Moel Tryfaen, Macclesfield and Gloppa; 

 but at the last-named, if not at the others, 

 we must assume the existence of steadily 

 alternating currents in the lakes in order 

 to explain the corresponding bedding of the 

 deposit. This, however, is not the only 

 difficulty. The "Irish Sea glacier" is sup- 

 posed to have been composed of streams 

 from Ireland, southwest Scotland and the 

 Lake District, of which the second fur- 

 nished the dominant contingent; the first- 

 named not producing any direct effect on 

 the western coast of Great Britain, and the 

 third being made to feel its inferiority and 

 "shouldered in upon the mainland." But 

 even if this ever happened, ought not the 

 "Welsh ice to have joined issue with the 

 invaders a good many miles to the north 

 of its own coast Y^^ Welsh boulders at any 

 rate are common near the summit of Moel 

 Tryfaen, and I have no hesitation in saying 

 that the pebbles of riebeckite-rock, far from 

 rare in its drifts, come from Mynydd 

 Mawr, hardly half a league to the east- 

 southeast, and not from Ailsa Craig.^* 



^' From Moel Tryfaen to the nearest point or 

 Scotland is well over a hundred miles, and it is a 

 few less than this distance from Gloppa to the 

 Lake District. In order to allow the Irish Sea 

 ice-sheet to reach the top of Moel Tryfaen the 

 glacier productive power of Snowdonia has been 

 minimized {Wright, " Man and the Glacial Epoch," 

 pp. 171, 172). But the difference between that 

 and the Arenig region is not great enough to make 

 the one incompetent to protect its own borderland 

 while the other could send an ice-sheet which 

 could almost cover the Clent Hills and reach the 

 neighborhood of Birmingham. Anglesey also, if 

 we suppose a slight elevation and a temperature 

 of 29° at the sea-level, would become a center of 

 ice-distribution and an advance guard to North 

 Wales. 



" The boulders of picrite near North Nobla, 



As such frequent appeal is made to the 

 superior volume of the ice-sheet which 

 poured from the Northern Hills over the 

 bed of the Irish Sea, I will compare in 

 more detail the ice-producing capacities of 

 the several districts. The present tempera- 

 ture of west central Scotland may be taken 

 as 47° ; its surface as averaging about 2,500 

 feet, rising occasionally to nearly 4,000 feet 

 above sea-level. In the western part of the 

 southern uplands the temperature is a de- 

 gree higher, and the average for altitude 

 at most not above 1,500 feet. In the Lake 

 District and the northern Pennines the tem- 

 perature is increased by another degree, 

 and the heights are, for the one 1,800 feet 

 with a maximum of 3,162 feet, for the other 

 1,200 feet and 2,892 feet. In north Wales 

 the temperature is 50°, the average height 

 perhaps 2,000 feet, and the culminating 

 point 3,571 feet. For the purpose of com- 

 paring the ice-producing powers of these 

 districts we may bring them to one tem- 

 perature by adding 300 feet to the height 

 for each degree below that of the Welsh 

 region. This would raise the average ele- 

 vation of central and southern Scotland to 

 3,400 feet and 2,100 feet, respectively; for 

 the Lake District and northern Pennines 

 to 2,100 feet and 1,500 feet. We may pic- 

 ture to ourselves what this would mean, if 

 the snow-line were at the sea-level in north 

 Wales, by imagining 8,000 feet added to its 

 height and comparing it with the Alps. 

 North Wales would then resemble a part 

 of that chain which had an average height 

 of about 10,000 feet above sea-level, and 

 culminated in a peak of 11,571 feet; the 

 Lake District would hardly differ from it; 

 the northern Pennines would be like a 

 range of about 9,000 feet, its highest peak 

 being 11,192 feet. Southern Scotland 

 would be much the same in average height 



from Llanerchymedd, though they have traveled 

 southward, have moved away much to the west. 



