BY H. I. JENSEN. 501 



In Brazil, alkaline rocks abound, bub their age is very uncertain. 

 However, here too, the work of O. A. Derby * seems to show 

 that a vast plateau-uplift occurred after the prolonged period of 

 depression and erosion of the Mesozoic, during which many 

 marine transgressions spread over the Brazilian area. The 

 Triassic and Cretaceous sediments there deposited have since been 

 largely removed by erosion, so that the alkaline rocks are left 

 surrounded by the rocks of the fundamental Archaean complex. 

 Their age can, therefore, only be stated as Post-Archaean, or in 

 some cases Post-Silurian or Post-Carboniferous. 



Tn general, it may be stated that alkaline extrusions have, 

 chiefly taken place along the borders of Mesozoic epicontinental 

 basins or transgressions which have been broadly uplifted without 

 much folding. Such is the case with the foyaitic rocks of 

 Arkansas, the Highwood Mountains of Montana, the Leucite 

 Hills of Wyoming, the Apache Mountains of West Texas, and 

 many other groups around the great American plains and arid 

 erosion peneplains of Cretaceous and Laramie sediments. Such, 

 too, is the case with the alkaline rocks of Abyssinia and Trans- 

 vaal in the faulted regions around the African steppes. The 

 alkaline rocks of Tyrol, Bohemia and Transylvania occur like- 

 wise in the faulted zone near the border of the elevated Mesozoic 

 basins of Central Europe; and, whilst those of Tyrol have been 

 estimated by most authors to be of late Mesozoic age, the other two 

 areas are Tertiary. The monzonitic rocks of Tyrol are, however, 

 only of moderate alkalinity, like some of those of the New 

 England plateau described by Card and Andrews. The monzo- 

 nitic magma from which all the basic and acid members of this 

 series are differentiated (Brbgger) is of such a character that it 

 might easily have originated by a calcic intrusion having stoped 

 in and assimilated at considerable depth a number of alkaline 

 Azoic and Palaeozoic rocks. 



The Norwegian alkaline area (Gran, Laugenthal and Nord- 

 marken) is generally considered to be a Palaeozoic intrusive mass. 



* Journal of Geology xv.-3. See also Q. J.G.S. Vol. xliii. No. 171. 



