534 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



late Palaeozoic and Mesozoic rooks of these parts we are reminded 

 of another remark of Suess, viz., " la force plissante a jadi agi 

 sur toute l'etendue du globe, tandis qu'elle est localisee ajourd 

 'hui dans des regions speciales." This localisation of the folding 

 force to geosyncline areas commenced in the Middle Palaeozoic. 

 Lapparent's map of the distribution of the Archaean, so widely 

 scattered about the globe, shows the same point (fig.l). Thus 

 Archaean and Palaeozoic lands have frequently become Mesozoic 

 and Neozoic seas and vice verscl. 



Those regions in which Mesozoic sedimentary rocks have 

 undergone no disturbance are mostly continental areas which 

 were only intermittently inundated by transgressions. When- 

 ever folded rocks of later age than Eopalaeozoic occur on conti- 

 nental areas the folding is of a superficial nature. Where 

 trachytic rocks occur associated with folded rocks the folding is 

 superficial. We have only to call to mind Gilbert's view that 

 the causes of movement are superficial in the Appalachians, deep- 

 seated in the Basin Ranges. The alkaline rocks of the United 

 States lie for the most part near and east of the former range. 

 So also the folding of the Carpathians is superficial, deepseated 

 in the Alps, superficial in the Jura, deepseated in the Pyrenees. 



The breaking-up of continents must unquestionably be the 

 result of secular contraction. By the gradual shrinkage of the 

 deeper contracting zones of the earth, the outer non-contracting 

 shell is in places left unsupported by the drawing inward of the 

 strata beneath it. This results immediately in great fractures 

 and subsidences of portions of the unsupported dome. Occa- 

 sionally when a cavity or macula is formed in this way, magmas 

 are squeezed into the hollow from adjoining sedimented areas. 

 These would be alkaline in nature, and might subsequently be 

 partly extruded on the fracturing of the crust. 



Having now reviewed the processes by which alkaline rocks 

 may have been derived, I will give a short sketch of the geogra- 

 phic distribution, geological association and age of all the chief 

 occurrences of alkaline rocks. The list is necessarily very incom- 

 plete, but each student can add to it for himself. 



