THE CENTURY'S PROGRESS IN GEOLOGY 



raneous with a period in Europe which in reality ante- 

 dated it by perhaps millions of year 



v 



All these puzzling features unite to render the subject 

 of historical geology anything but the simple matter the 

 fathers of the science esteemed it. No one would now 

 attempt to trace the exact sequence of formation of all 

 the mountains of the globe, as Elie de Beaumont did a 

 half-century ago. Even within the limits of a single 

 continent, the geologist must proceed with much caution 

 in attempting to chronicle the order in which its various 

 parts rose from the matrix of the sea. The key to this 

 story is found in the identification of the strata that are 

 the surface feature in each territory. If Devonian rocks 

 are at the surface in any given region, for example, it 

 would appear that this region became a land surface in 

 the Devonian age, or just afterwards. But a moment's 

 consideration shows that there is an element of uncer- 

 tainty about this, due to the steady denudation that all 

 land surfaces undergo. The Devonian rocks may lie at 

 the surface simply because the thousands of feet of car- 

 boniferous strata that once lay above them have been 

 worn away. All that the cautious geologist dare assert, 

 therefore, is that the region in question did not become 

 permanent land surface earlier than the Devonian age. 



But to know even this is much sufficient, indeed, to 

 establish the chronological order of elevation, if not its 

 exact period, for all parts of any continent that have 

 been geologically explored understanding always that 

 there must be no scrupling about a latitude of a few mill- 

 ions or perhaps tens of millions of years here and there. 

 K 145 



