REFERENCE TO THE ANTIQUITY OP MAN". 



403 



6000 years is obtained. But the growth of peat varies extremely. 

 It may be, in some cases, not more than 1 foot in a century, but it is 

 commonly more, being sometimes as much as 4, 5, and even 10 feet 

 in that time ; and while it is estimated that to form 1 foot of coal, 

 from 2| to 3 feet of woody matter may be required, it is clear that 

 lignite, which has lost less of its original constituents * than coal, and 

 of which the specific gravity is about 1*25, while that of coal is about 

 1*5, cannot require for each foot 5 feet of peat and wood. Taking, 

 therefore, the original thickness of the peat at 24 instead of 60 feet, 

 and the growth at 4 feet in a century, 600 years, instead of 6000, 

 would be sufficient for the formation of the Diirnten beds. 



These intervals, therefore, although they may involve considera- 

 tions respecting hundreds, are scarcely likely, as they must have been 

 subordinate to the general progress of the ice-sheet, to involve ques- 

 tions relating to thousands of years. It is to be observed also that 

 there is no evidence in North America of an interglacial period in 

 the sense of the one supposed to have existed in Europe, although 

 there is evidence that after the great ice-sheet had retreated for a 

 very considerable distance northward, there was a pause or a partial 

 advance again southward — an advance marked this time by deeply 

 lobed lines of moraines. 



Whilst there are these reasons for prolonging the duration of the 

 Glacial epoch, there are other factors in the question which tend to 

 shorten it. At present the discharge of ice from the Greenland 

 sheet is merely the surplus under conditions of a settled mean an- 

 nual temperature ; but the Glacial epoch was a time, on the whole, 

 although there may have been pauses, of constantly increasing cold, 

 and of constant increase in the area of the great ice-sheet, and there- 

 fore there was not merely a supply due to a uniform mean annual 

 temperature, but the increments arising from the gradual secular 

 refrigeration. 



It may also be a question whether or not the rainfall was then 

 greater than now. At present in Greenland it is small, apparently 

 under 20 inches, while in the North-American old ice-area it is not 

 less than from 40 to 45 inches annually. Possibly the precipitation in 

 the Glacial epoch was even larger, for the Florida promontory, which 

 now deflects and contracts the Gulf-stream, was at that time con- 

 siderably smaller, the coral reef by which it is formed not having 

 then extended so far south. Consequently the channel through 

 which the Btream passed being wider, a greater volume of water 

 flowed through ; and this large body, thus carried into the North 

 Atlantic, moving probably with greater velocity and having a higher 

 temperature than now, may, in consequence of the greater satura- 

 tion of the incumbent air, have materially affected the precipitation 



* In the extreme case of the conversion of wood and peat into anthracite, in 

 which the proportion of oxygen and hydrogen to the carbon is as 5 : 95, the esti- 

 mate is of from 7 to 8 feet of wood to 1 foot of anthracite ; and in ordinary coal, 

 where these constituents are roughly as 15 : 85, the estimates vary from 2^ to 

 3 feet of woody matter to 1 foot of coal. In lignite, then, where the change has 

 involved less loss (say to 30 O -}- H : 70 O) and the pressure has been less, the 

 ..compression must certainly have also been less. 



