801 



Movements on a large scale will give rise to overlliiiist sheets, 

 where one mass of rock has been pnshed bodily over anotliei'; the 

 earth's crust in situ will increase in thickness, an additional reason 

 for the stopping up of the volcanic vent. A new waj' is opened 

 for the magma to reach the surface along the thrust-planes; most 

 often the magma, if it reaches the surface will appear on a lower 

 level i.e. in the region here discussed below the surface of the sea 

 along the outer margin of the row of islands and movements in the 

 direction of the "Vorland" will cause the volcanic products to be 

 gradually overlain by the moving masses. 



Disruption of the strata nmy occur abruptly without any folding. 

 It goes without saying that in this case there is no question about 

 an exit for the magma on the tops of the anticlines ; the disturbance 

 of the equilibrium caused by the movements in the eaith's ciust are 

 directly attended with an increase of thickness of the crust where 

 the crustal movements take place. The above shows sufficiently that 

 the magma can reach the surface while folding is in progress, but 

 that the place where and the time when volcanic activity will 

 appear, depend on the character of the crustal movements. 



The magma can find an egress also without the aid of crustal 

 movements. By assimilation of adjacent rocks or by magmatic stoping ^) 

 the njagma can force its way upwards and extrude by de-roofing, 

 as for instance Daly*) assumes for the rhyolite-plateau of the 

 Yellowstone National Park, Ussing*) for the Greenland inliusions 

 and myself^) for the intrusion of the Pilandsberg in the Transvaal. 



This volcanic activity may manifest itself parlicidarly in the 

 intermittent periods of rest of crustal movements. Secondly, the 

 magma will only be able to penetrate through the crust in places 

 where it is comparatively thin, because otherwise it will have cooled 

 down too much and have lost much of its mobility. Consequently 

 no effusion can be expected where the crustal movements have 

 engendered a thickening of the earth's crust; this will then more 

 likely be possible along the margins of the anticlines, particularly 

 along the injier ones. 



') R. A. Daly, Igneous rocks and their origin, 1914. p. 194. 



2) Ibid, p. 122. 



*) N. V. UssiNG, Geology of the country around Julianehaab, Greenland, Medde 

 lelser om Gronland. Vol. XXXVIII and Mus. de Min. et de Géol. de l'Université 

 de Gopenhague. Comm. Geol. N^. 2. 1911. 



*) H. A. Brouwek, On the origin of primarv parallel structure in lujaurites. 

 Proc. K. A. V. W., 8 Nov. 1912. 



Ibid. On the geology of the Alkalirocks in llie Transvaal. Journ. of Geol. XXV, 

 1917, p. 768. 



