572 THE DISTRIBUTION, ETC., OP ALKALINE ROCKS, 



doleritic constituents occur in bunches of fragments which are 

 relics of a dolerite which has been assimilated by the alkaline 

 magma. The xenogenic nature of the fragments and phenocrysts 

 shows us clearly that an alkaline magma has burst through a 

 partially consolidated doleritic mass, mixed with it and to a degree 

 assimilated it. 



It seems that the trachy-andesite magma could very well have 

 originated in the same way. The trachy-andesites and melano- 

 cratic trachytes and phonolites frequently contain corroded 

 olivines, augites, etc., which have a xenogenic appearance. 



The corundum basalt (Billy King's Creek, W.40) referred to 

 above may itself be formed from a basalt-magma by assimilation. 

 In the field this rock rapidly changes into a more or less typical 

 olivine basalt with pilotaxitic fabric, and exhibits great variations. 



Mixture or assimilation has therefore probably played an 

 important part in the production of the Warrumbungle rocks. 

 The question therefore arises how the trachy-andesite magma 

 could have originated by the aid of these processes. 



The lateral flow of alkaline rocks from oceanic basins to con- 

 tinental areas during which the primitive alkaline precipitates 

 were changed to a foyaitic magma would have commenced in this 

 part of the world in the Carboniferous. During the late Palaeo- 

 zoic sedimentation of New England, alkaline magmas were 

 thus forced aside and accumulated in the shear-zone of adjacent 

 continental areas. This movement would actively continue 

 through the Mesozoic era when the New England district was 

 influenced by rise of isogeotherms. Sedimentation would pro- 

 bably also tend to force in a lateral direction some underlying 

 basic magmas, and more or less mixing might take place. The 

 alkaline magmas belonging to a higher plane in the zone of shear 

 would, on the fracturing and collapse of a continent, be the first 

 to be expelled (fig. 8). 



If we now assume first that no mixture with basic rock has 

 taken place in the process of lateral squeezing we will in the 

 cataclysmal period have first acid alkaline eruptions forming 

 mamelons, laccolites and breccia-cones, and next, after a pausa 



