Fairbanks.] 



Geology of Point Sal. 



23 



seems to the author might properly be termed "segregated veins," 

 are often finer grained than the exterior. Analcite is abundant in 

 large masses sometimes, two inches in greatest diameter, either 

 completely or partly filling spaces apparently left between the 

 other constituents, in the latter case being well crystallized. These 

 spaces sometimes occupy the centers of the bunchy areas, in 

 which there is a gradual increase in coarseness from the normal 

 toward the center. The centers of the coarse crystallization, now 

 empty or filled with analcite, might conceivably have been left as 

 spaces and subsequently filled, or possibly occupied by a residual 

 portion of the magma, which the author believes is represented by 

 the analcite scattered through the rock in the polyhedral areas. 

 There is another set of veins, or more properly dikes, which are 

 undoubtedly of subsequent intrusion. This took place probably 

 while the mass was still hot, for they are fairly coarse grained, 

 though only a few inches in diameter. They are similar in appear- 

 ance to the lighter portions of the rock, though occurring only in 

 the darker. Sometimes distinct clearly defined nodules appear 

 which seem like inclusions. One type of the rock is almost jet 

 black, with lustrous augites and feldspars. Angular pores are 

 numerous and lined with a soft decomposition product. They 

 must be due to the partial removal of some constituent. Analcite 

 is not visible to the unaided eye in the most of the darker facies of 

 the rock, although its original presence is clearly indicated by the 

 surface weathering. It seems probable that some of the larger 

 cavities occupied by the analcite may be of secondary origin, but 

 this question is difficult of solution. 



Microscopic Petrography. — Many specimens were obtained 

 from the large boulder-like masses at the foot of the cliffs which 

 were apparently very fresh, and it was hoped that in some of them 

 direct evidence could be obtained of the primary condition of the 

 rock, for it is believed that the analcite, everywhere so abundant, has 

 replaced a soda rich silicate, probably nepheline. Although many 

 sections were prepared, no nepheline could be found, but there was 

 accumulated a mass of evidence, indirect in its nature, yet still of such 

 importance that it seems to the author that there can no longer be 

 any doubt that the analcite has replaced a primary soda mineral. 

 3 



