Fairbanks.] 



Geology of Point Sal. 



87 



rocks of one period of eruption are the differentiation products of a 

 common magma basin. The differentiation is brought about by a 

 diffusion according to the order of crystallization toward the outer 

 surface. The more remote the periods of eruption, the more differ- 

 entiated should be the products erupted. . . . The series of 

 surface rocks is not necessarily the same as that of the deep-seated 

 rocks. These are in large part controlled by secondary differentia- 

 tion, being a repetition of basic, acid, basic, etc." He says that 

 most authors have not made this distinction. In conclusion he 

 adds : " In general, the farther the magma basin is removed from the 

 magma trunk, the greater the amount of differentiation which has 

 taken place, so much less regularity will there appear in the series 

 of eruptions. Any irregularity or departure from the rule in the 

 deep-seated rocks must be all the more pronounced in the surface 

 rocks." Brogger also emphasizes the point that his sequence is 

 that for regions removed from mountain-making influences. 



In many respects the views of the investigators quoted do not 

 differ greatly. It would seem, however, that in the majority of 

 instances there are complicating causes at work which so affect the 

 series of eruptions that no statement can be formulated which will 

 hold true in all. Iddings' view of continually greater extremes in 

 the character of eruptions of any one period from a common reser- 

 voir is only one aspect of the truth, of which that of Teall and 

 Brogger, of a continually increasing acidity, is another. In moun- 

 tain regions, where disturbances are taking place, the sequence 

 must of necessity be different in many cases from that which would 

 occur theoretically in a magma and its apophyses when not so 

 affected. 



At Point Sal there can be observed both kinds of petrological 

 complexity described by Geikie and Teall, differentiation in situ and 

 successive intrusion. The gabbro and peridotite were each fully 

 differentiated in some deeper-seated region, while in each secondary 

 differentiation took place during the movement upward or after 

 attaining its present position. Judging from the surface exposures 

 the gabbro can not form more than one-fourth of the total mass, 

 and it may be less, so that the common magma must still have been 

 very rich in magnesia. Owing to the fact that not all the different 

 7 



