CHANGES OF CLIMATE DURING- GEOLOGICAL PERIODS. 79 



of till. It has been toaced for more than forty miles along the coast , and 

 consists of stumps of trees standing erect. Above these we have a fluvio- 

 marine series, the flora and fauna of which belong to a warm inter- 

 glacial period. Above this comes the Norwich boulder till, containing 

 blocks 6ft. or 8ft. in diameter, many of which must have come from 

 Scandinavia. Then, in succession, (1) beds of sand and gravel ; (2) a drift 

 yielding shells indicating a mild climate ; (3) the upper boulder clay ; (4) 

 freshwater beds, containing seeds, bones, &c, indicating a mild and 

 temperate condition of climate. 



Professor Geikie observed a section of a cliff in the South of Scotland 

 which stood thus: — 1, vegetable soil ; 2, boulder clay 30ft. to 40ft.; 3> 

 yellowish gravelly sand ; 4, peaty silt and clay ; 5, fine ferruginous sand ; 

 6, coarse shingle 2ft. to 3ft. ; 7, coarse stiff boulder clay loft, to 20ft. 



In one instance, at Diirnten, on the lake of Zurich, a coal seam 12ft. 

 thick, which would require from six to ten thousand years for its forma- 

 tion, is found lying on boulder clay. Overlying this coal is another mass 

 of drift and clay, 30ft. in thickness, with rounded blocks, and on the top 

 of this upper drift lie long angular erratics, which evidently have been 

 transported on the back of glaciers. Indeed the formation of coal beds 

 can be explained on the supposition of interglacial periods, as it can be 

 on no other. An interglacial climate is the one best suited for the growth 

 of the coal plants. The coal period indicates the existence of a moist, 

 equable, and temperate climate, such as will be shown must have been 

 the character of our climate between these glacial epochs. At the same 

 time, we have in the cold periods of such an epoch the condition most 

 favourable for the preservation of those plants, for then they would be 

 submerged and covered over by a thick deposit of sand, mud, and clay. 

 In South Africa there is evidence not only of a glacial condition during 

 the Pliocene period, but also of a warmer climate than now prevails in 

 that region. Hecent glaciation extends over a large portion of Natal and 

 other parts. 



It is not to be wondered at that the evidence of glacial action in very 

 remote times should be very scanty. Indeed, remembering what must 

 be the effects of the warm periods succeeding, it is, perhaps, remarkable 

 that we have any evidence left of the preceding cold ones. It is evident 

 that cold periods are best marked in temperate regions, and warm ones 

 in Arctic ; and in many Arctic localities the remains of a luxuriant flora of 

 the Miocene period have been often found in great abundance. Remains of 

 mammalia are found in the icy alluvial deposits of North Siberia, and 

 unfossilized trees — conifers and others — in the Arctic lands of North 

 America. Coal beds of the carboniferous age are extensively developed 

 in Arctic regions, proving that a mild and temperate condition of climate 

 must in some part of the carboniferous age have prevailed up to the very 

 pole. Masses of Silurian limestones are also found in the Arctic regions, 

 spread over a wide surface containing encrinites, corals, and rnollusca, 

 and other fossil remains. 



In the Eocene period, when palms and turtles and crocodiles 

 inhabited England, we find travelled blocks and other indications of ice- 



