502 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



It is known to be Post-Silurian, and probably Post-Devonian, but 

 nothing more definite is known. It is certain from the grain- 

 size of the nordmarkites, foyaites, laurdalites and laurvigites 

 of the region that they consolidated at a considerable depth. It 

 is therefore possible that these rocks too were intruded as late as 

 the Eocene or late Cretaceous, and that Tertiary erosion has 

 exposed them; for we know well that a vast amount of erosion 

 can be effected in a long period like the Tertiary; and the 

 researches of Hull and Spencer* would lead one to believe that 

 Norway and much of Europe in late Tertiary and Post-Tertiary 

 times stood 8,000 feet higher than at present, hence the power of 

 glacial and pluvial erosion must have been far greater than at 

 present. 



At whatever age these Christiania foyaitic rocks may have 

 been intruded, the fact remains that their district is the Scandi- 

 navian earthquake epicentre at the present day. It is also clear 

 that it is situated on the faulted flanks of the Scandinavian horst 

 and near the border of the Mesozoic basin of sedimentation which 

 extended over Denmark, South Sweden and Central Europe. 



The similarity of lithological associations and of stratigraphical 

 relationship between all areas where alkaline rocks abound, is 

 very striking: — they occur always near and sometimes within the 

 massifs or horsts adjoining great plains of Mesozoic sediments. 

 Therefore they occur in the very places where we should expect 

 the greatest production of heat by the differential movement of 

 great blocks severed by faulting, where some great masses of the 

 earth's crust are drawn down and partially remelted, being 

 crushed between other segments which remain stationary or are 

 being elevated. 



In many parts of Australia, the United States, Brazil and 

 Eurasia we have Mesozoic sediments capping the horsts. 



The age of the alkaline rocks is often very difficult to fix, but 

 in almost all cases where a reliable determination has been found 



* For references see The American Geologist, Vol. xxxv. No. 3, p. 152 ; 

 also Ch. iv., 'The Evolution of Earth Structure,' by T. Mellard Reade. 



