Fairbanks.] 



Geology of Point Sal. 



8i 



row band of feldspar, then hornblende with some feldspar, followed 

 by narrow bands of feldspar not strongly marked. The augite 

 occurs in a band free from the other constituents, having a predom- 

 inating number of the crystals with their vertical axes approxi- 

 mately normal to the walls. 



Banding is also exhibited in an interesting manner by many of 

 the narrow basic dikes which are among the latest of the intrusions. 

 It is due mainly to the unequal distribution of the feldspar. It is 

 hard to understand how the banding in this case can be due to aught 

 else than movement in an originally homogeneous magma, in con- 

 junction with a tendency to segregation and slight differences in 

 time of consolidation of the different components. 



The dike of serpentine in the gabbro west of Point Morrito 

 which contains the abundant inclusions of gabbro has already been 

 mentioned. The inclusions range in size from three inches in great- 

 est diameter down to mere specks, and are so arranged as to give 

 the appearance of banding. It would seem that here is a clue to 

 one of the causes of the phenomenon under consideration. The 

 inclusions would play the part, during movement, of constituents 

 already solidified. • The body of the dike which consists wholly of 

 diallage and decomposed olivine is distinctly banded in several 

 places by lighter colored feldspathic layers. These are quite 

 sharply bounded and it is quite possible that they are due to the 

 mixing of two heterogeneous magmas. 



The large body of feldspathic wherlite or picrite inclosed in the 

 gabbro presents a linear arrangement of the feldspathic aggregates 

 which are dotted thickly through it. These aggregates are not 

 massive but consist of feldspar and serpentine in about equal pro- 

 portions. They are frequently drawn out in lenticular patches two 

 or three inches long. Segregation must have been contempora- 

 neous with movement. In the peridotite at Point Morrito there are 

 areas in which the diallage and olivine have been segregated in 

 large bunches, but unaccompanied with movement. In other spots 

 there are irregular patches of picrite in a matrix consisting chiefly 

 of olivine. In both of these cases a differential movement at the 

 time of consolidation would have produced banding. 



The feldspathic streaks occurring in the dunite and other varie- 



