Fairbanks.] 



Geology of Point Sal. 



A substance which is undoubtedly the decomposition product 

 of olivine is present in several of the slides, prepared from the darker 

 facies of the rock. It varies in color from reddish brown to green. 

 The polarization colors are quite high, and the extinction is par- 

 allel to the cleavage cracks. 



Magnetite is fairly abundant in the rock. The ophitic variety 

 generally contains less than the panidiomorphic. In the latter it 

 has quite generally been formed subsequent to both feldspar and 

 augite, being allotriomorphic with reference to them (Plate 2, 

 Fig. 3). Masses of magnetite are frequently aggregated in certain 

 portions of the section in a manner precisely similar to that of the 

 augite in the ophitic structure. The grains fill angular spaces 

 between the small lath feldspars, each group probably being referable 

 to one individual. In the ophitic facies it often crystallized between 

 the periods of the feldspar and augite. A remarkable aggregate of 

 skeleton forms was observed lying partly within an augite crystal 

 and partly in a mass of decomposed feldspar and analcite. The 

 magnetite differs from that of the Cuyamas eruptive in that titanium 

 is absent. 



Analcite is by far the most important of the secondary products 

 present in the rock. Although it is not visible to the unaided eye 

 in many of the hand specimens, yet the microscopic examination 

 shows that it was probably once present in all. In places it is now 

 represented by other secondary products which have often been 

 more or less leached out, giving the rock a porous aspect. In many 

 slides it is shown to have replaced the feldspars to a considerable 

 extent, being scattered through them in the most irregular manner. 

 The difference in composition in the different portions of the feldspar 

 crystals seems to have no effect upon the degree of alteration. In 

 one crystal it may be more pronounced in the central part; in another 

 the outer shell has undergone the greatest amount of alteration. 



The most interesting question is raised by its occurrence in the 

 polyhedral or wedge-shaped areas between the other components. 

 Upon the correct determination of the significance of its presence 

 here rests to a large extent the classification of this peculiar rock 



type- 

 In the light-colored, more coarsely crystalline portions of the 



