Ch. XL] NORFOLK DRIFT, ETC. 131 



now inhabit more northern seas, where we may, perhaps, hereafter find 

 living representatives of some of the unknown fossils. The distance to 

 which erratic blocks have been carried southwards in Scotland, and the 

 course they have taken, which is often wholly independent of the present 

 position of hill and valley, favors the idea that ice-rafts rather than gla- 

 ciers were in general the transporting agents. The Grampians in For- 

 farshire and in Perthshire are from 3000 to 4000 feet high. To the 

 southward lies the broad and deep valley of Strathmore, and to the 

 south of this again rise the Sidlaw Hills* to the height of 1500 feet and 

 upwards. On the highest summits of this chain, formed of sandstone 

 and shale, and at various elevations, are found huge angular fragments 

 of mica-schist, some 3 and others 15 feet in diameter, which have been 

 conveyed for a distance of at least 15 miles from the nearest Grampian 

 rocks from which they could have been detached. Others have been 

 left strewed over the bottom of the large intervening vale of Strath- 

 more. 



Still farther south on the Pentland Hills, at the height of 1100 feet 

 above the sea, Mr. Maclaren has observed a fragment of mica-schist 

 weighing from 8 to 10 tons, the nearest mountain composed of this for- 

 mation being 50 miles distant.f 



The testaceous fauna of the boulder period, in Scotland, England, and 

 Ireland, has been shown by Prof. E. Forbes to contain a much smaller 

 number of species than that now belonging to the British seas, and to 

 have been also much less rich in species than the Older Pliocene fauna 

 of the crag which preceded it. Yet the species are nearly all of them 

 now living either in the British or more northern seas, the shells of more 

 arctic latitudes being the most abundant and the most wide spread 

 throughout the entire area of the drift from north to south. 



This extensive range of the fossils can by no means be explained by 

 imagining the mollusca of the drift to have been inhabitants of a deep 

 sea, where a more uniform temperature prevailed. On the contrary, 

 many species were littoral, and others belonged to a shallow sea, not 

 above 100 feet deep, and very few of them lived, according to Prof. E. 

 Forbes, at greater depths than 300 feet. 



From what was before stated it will appear that the boulder formation 

 displays almost everywhere, in its mineral ingredients, a strange hetero- 

 geneous mixture of the ruins of adjacent lands, with stones both angular 

 and rounded, which have come from points often veiy remote. Thus we 

 find it in our eastern counties, as in Norfolk, Suffolk, Cambridge, Hunt- 

 ingdon, Bedford, Hertford, Essex, and Middlesex, containing stones from 

 the Silurian and Carboniferous strata, and from the lias, oolite, and chalk, 

 all with their peculiar fossils, together with trap, syenite, mica-schist, 

 granite, and other crystalline rocks. A fine example of this singular 

 mixture extends to the very suburbs of London, being seen on the 

 summit of Muswell Hill, Highgate. But south of London the northern 



* See above, section, p. 48. f Geol. of Fife, &c. p. 220. 



