Fairbanks.] 



Geology of Point Sal. 



35 



the supposition that they represent a primary constituent altered to 

 analcite. 



If there is a strong improbability of the existence of original 

 miarolitic spaces in this rock, the view of the subsequent introduc- 

 tion of the analcite is thereby weakened. For the latter view to 

 have any probability at all, it is first necessary to demonstrate the 

 probability of the existence of a porous rock. 



There is an objection, however, of considerable weight of a 

 wholly different nature, which might be used against the views just 

 presented. This concerns the chemical composition of the augite. 

 It may be argued that a magma which was originally rich in soda 

 would necessarily have produced an aegirine or an aegirine-augite, 

 and that with the presence of a normal basaltic augite, there is 

 good reason to believe that the magma contained but little soda. 

 There is the fact also that the augite has apparently the same char- 

 acter whether its period of crystallization was coeval with or fol- 

 lowed that of the feldspar. In the poikilitic facies the feldspars are 

 all small, and do not show zonal extinction, and it is possible that 

 in the two types the relative periods of crystallization did not 

 differ as much as might be expected from the marked difference in 

 structure. 



It is a fact which has been emphasized more perhaps by Brogger 

 than any one else, that the aegirine molecule plays an important 

 part in the composition of the augites of the nepheline bearing rocks. 

 The question whether it is invariably present in the augites of such 

 rocks is an important one, and would have great weight in disprov- 

 ing the author's views if it should appear that such is the case. 

 Rosenbusch* says: "All nepheline rocks contain a basaltic augite 

 as the most important component besides nepheline. In nephe- 

 line rocks the augite is characteristically accompanied by a red 

 brown to blood red biotite, more rarely by aegirine or amphibole 

 minerals." In the last edition of his " Mikroskopische Physio- 

 graphic,"! he says: "Akmite and aegirine appear to be confined 

 entirely to the eruptive rocks, and especially to arise in the magmas 



* Mikroskophische Physiographic der Massigen Gesteine, 2d edition, p. 



782. 



t Ibid., Vol. II, 3d edition, p. 538. 



