Fairbanks.] 



Geology of Point Sal. 



67 



grains of magnetite are abundant. All the minerals of the rock are 

 fresh. There are a few large crystals of augite showing the diallagic 

 cleavage. 



A fine-grained dike, three inches wide, is intrusive in the gabbro 

 and peridotite west of Point Morrito. A thin section shows horn- 

 blende, augite, hypersthene, and feldspar, the minerals all being in 

 a fresh condition. A distinct banding is shown, due to the segrega- 

 tion of a part of the hornblende. The four constituents are present 

 in about equal proportion, while magnetite is abundantly scattered 

 through the rock. Although the most of the constituents are 

 granular, there is a tendency to the elongation of some of the 

 feldspars and their arrangement with their long diameters parallel 

 to the walls of the dike. No other instance of a parallelism of this 

 kind appeared. 



In a dike of crushed serpentine south of the Head occur nodules 

 of a dark, fine-grained rock, with serpentine adhering to one side. 

 They appear to belong to a dike broken up in the movement which 

 brought about the crushing of the serpentine. The composition of 

 this rock is the same as the last. There is a slight tendency toward 

 the arrangement of the constituents in lines. This is particularly 

 noticeable in the case of the magnetite, which appears in bands made 

 up of scattered grains. 



The combination of minerals just described is not a common 

 one for the coarse gabbro, olivine being so widely distributed 

 through that rock that but few specimens were obtained which did 

 not contain it. The combination of feldspar and augite or diallage 

 is also uncommon in the coarser rocks. 



Norite-gabbros are not abundant, though several dikes of this 

 rock occur. Hornblende almost always is present in these dikes, 

 which are most numerous in the gabbro-peridotite area west of Point 

 Morrito. A small dike only a few inches wide, and almost apha- 

 nitic in the hand specimen, might be cited as a characteristic example. 

 Under the microscope it is seen to possess a typically holocrystal- 

 line granular structure. The components, feldspar, augite, and 

 hypersthene, are perfectly fresh and present in about equal propor- 

 tions. Magnetite dust is scattered thickly through the slide. 



The typical norite, free from augite, was found to occur only as 



