584 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



described a nosean-felspar rock from Taimyr River, N. Siberia, 

 supposed to be of the same age.* 



Africa — None. 



America — The Montreal ali-syenites appear to be Palaeozoic. 

 The alkaline rocks of Pigeon Point, New Jersey, Lake Champlain, 

 and many other places in the North-eastern States are still more 

 indefinite as regards age. Those of Brazil are supposed to be 

 Permo-Carboniferous. In no single instance have we definite 

 proof except when they are known to be Post-Cretaceous. 



In the vast majority of instances the alkaline rocks belong to 

 the period of Tertiary convulsions, but there is no reason why a 

 similar period of disturbance should have produced alkaline 

 magmas in the Palaeozoic. 



In no single case in any continent do the alkaline rocks occur 

 in a range of intensely folded Mesozoic and Tertiary sediments, 

 such as might have constituted a deep sea throughout the late 

 Palaeozoic and Mesozoic periods. The Mesozoic sediments repre- 

 sented in their vicinity are relics of mere transgressions or epi- 

 continental seas spreading over subsiding portions of continents. 



The alkaline rocks are always on or near portions of continental 

 masses which have been fractured by faults and developed 

 trough-faults. 



Evidence has been given for all these generalisations which 

 may be considered tentatively proved. 



Further, it appears that alkaline rocks are seldom erupted in a 

 subsiding Riickland, almost always in a subsiding Vorland. 



BRIEF SUMMARY OF CONCLUSIONS. 



1. Alkaline rocks are practically confined to the flanks of old 

 plateaux (continental areas) which have escaped heavy sedimen- 

 tation in late Palaeozoic and Mesozoic times. 



2. They occur on fissure-lines along which great crustal move- 

 ments took place that led to the dismemberment of the plateaux. 



*Bull. Acad. Imp. Sci. St. Pet. New Series, iii. p.421. 



