LAND AND WATER 19 



This work of orogenesis, or mountain building, characterizes 



the great geological epochs, and the formation of one series of 



folds generally required an entire epoch for its consummation. 



The era in which the Huronian chain was formed is generally 



known as the pre-Cambrian ; while the era extending from the 



origination of the Caledonian folding to the completion of the 



Hercynian, is known as the Primary. A long period of relative 



calm — the Secondary — followed. Orogenic disturbances 



continued in Tertiary times, and resulted in the formation of 



the Alpine and Alpine-Himalayan folds. Strictly speaking, 



one might concede that this third period has not yet come to an 



end, since the orogenic movement characteristic of it still 



continues. We can, indeed, point to movements indicative of 



the rising up of the land on many of our coasts, as on the 



Saintonge coast ; x or of its sinking, as in the Bay of 



Douarnenez. Earthquakes frequently occur at those points 



of the globe clearly connected with the intersection of mountain 



chains ; volcanic craters are numerous, active and quite 



evidently associated, in regions where levelling is still 



proceeding. But the present, or Quaternary Epoch, was 



marked by an event to which we naturally attach the greatest 



importance, namely Man's effective appearance as the master 



of the earth ; the beginning of this dominance coincides with 



a climatic condition which is regarded as closing the tertiary 



epoch — a lowering of the temperature which over and over 



again permitted a prodigious extension of glaciers. This 



glacial period was unquestionably the consequence of tertiary 



orogenic phenomena, which, by raising high peaks on the 



surfaces levelled during Secondary times, and by modifying 



the distribution of continents and oceans, favoured the 



accumulation of great masses of snow, augmented every winter 



on the summits of the vast chains of newly formed mountains. 



From a geological view-point, no new factor was involved, 



except perhaps at the beginning of the period of erosion 



of the Alpine chains. But we naturally attach particular 



importance to phenomena so intimately linked with our own 



history, and all geologists, for that reason, regard the era 



in which the human species began to assume an important 



1 The movements found in such regions, it is true, are rather equilibratory, 

 and belong to the category of epirogenetic movements, thanks to which the 

 sea covers zones of subsidence which it abandons and re-occupies alternately. 



