Fairbanks.] 



Geology of Point Sal. 



4i 



ical form is assumed. These peculiar forms are often not sharply 

 differentiated from the structureless basalt. The marked features 

 of the ropy or spheroidal masses are only brought out distinctly 

 where erosive action has been favorable. They vary in size from 

 a few inches up to three feet in diameter. Where the clean sur- 



Figure 3. — Spheroidal basalt. 



faces appear above the reach of the heavy waves, the structure is 

 often apparent. The whole body of a spheroid is in most cases 

 highly amygdaloidal. The outer portion appears to be more 

 resistant, and consequently when they are broken into, a thin shell 

 is left around a partially hollow interior. The outer shell, though 

 fully as amygdaloidal, is more compact and the amygdules smaller. 

 The little rolls are sometimes perfectly massive and structureless, 

 although filled with calcite seams. The amygdules in the basalt 

 are often an inch in diameter and are partly or wholly filled with 

 quartz and epidote. When only partly filled the cavities are lined 

 with drusy crystals. Dikes of an andesitic lava which are slightly 

 amygdaloidal cut through the basalt in the most irregular manner. 

 One dike, of very limited extent but exceptionally decomposed, 

 showed large porphyritic feldspars and contained inclusions of a 

 coarsely amygdaloidal lava. 



