138 DISCUSSIONS IN CLIMATOLOGY. 



passing from perihelion round to perihelion to 23,230 

 years, which will represent the mean interval between 

 two consecutive intergiacial periods. But as the 

 motion of the perihelion was very irregular, the length 

 of the interval between the periods would, of course, 

 differ considerably. 



When we consider how difficult it must be to detect 

 in the drift covering glaciated countries even a relic 

 of early intergiacial deposits, and when, moreover, we 

 remember that it is only within the past few years 

 that geologists have begun to bestow any attention on 

 the subject, it is certainly not surprising that direct 

 geological evidence of so few intergiacial periods has 

 as yet been discovered. In England, geologists have, 

 however, already detected evidence of three intergiacial 

 periods, with four or five ice -periods. In Germany, 

 quite recently, two intergiacial periods and three or 

 more ice-periods have been recognised by competent 

 observers. In Denmark there are four boulder-clays 

 separated by intercalated beds of sand and clay. In 

 severely -glaciated Scotland, where traces of former 

 intergiacial periods can hardly be expected, there have 

 nevertheless been found in old preglacial buried 

 channels, and other sheltered hollows, three, four, and 

 in some places five, boulder-clays, separated from one 

 another by immense beds of sand, gravel, and clay. 

 Some of these beds are found to be continuous for long- 

 distances. It is true that these intercalated beds have 

 yielded few or no organic remains, but it may well be 

 that further research will yet result in the discovery 

 of more abundant fossils ; for frequently the beds in 

 question are too thick and too extensive to allow us 

 to infer their subglacial origin. They do not in 

 such respects resemble the deposits which have been 

 accumulated by aqueous action under ice, but have 



