i.9 CLIMATIC CHANGES 15 



period, the second inter-glacial. As we go farther back the study 

 becomes more and more difficult, but the available evidence suggests 

 that fluctuations of climate considerable enough to alter the entire 

 fauna and flora may have taken place at a periodicity of something 

 over 40,000 years. It is a measure of the difficulty of geological 

 science that we cannot yet give a systematic account of the chronology 

 or climatic changes even of the relatively recent Pleistocene period 

 (variously estimated at 600,000 to 1,800,000 years) during which these 

 glaciations occurred. 



RM590 R.M550 

 EGI 1 EGI 2 



EARLY GLACIATION 



RM476 RM 435 



ApGI I ApGI 2 



ANFEPENULTIMATE GLACIATION 



Fig. 2. Curve of solar radiation received at 65 ° N. lat. in the summer. The radiation is 

 expressed in 'canonic units' (related to the solar constant in calories). Time in thousands 

 of years. R.M. 25, &c, indicate the radiation minima. (From Zeuner, based on the tables 



of Milankovitch.) 



As we proceed to study times still more remote our vision becomes 

 increasingly blurred. We can now only rarely distinguish periodicities 

 as short as 40,000 years, though there is evidence that they existed, for 

 instance from varved Cretaceous sediments. All we can see in the 

 study of geological deposits are the very marked changes produced by 

 the major movements of orogenesis and by the isostatic readjust- 

 ments. The surprising thing is that these immensely slow changes 

 have been sufficiently regular to leave layered deposits, allowing the 

 development of a system of geological classification. The process of 

 sedimentation was interrupted by periods when the continental shelf 

 on which the rocks rest was raised above the water surface and under- 

 went denudation for a while, before being again lowered below the sea 

 and covered with a new deposit. During the interval, while the shelf 

 was raised above the water, the animals and plants in the sea became 



