Fairbanks.] 



Geology of Point Sal. 



lower limit of the gypsum-bearing clays. In a branch of the first 

 canon east of the Chute, it forms a great cliff, for some distance 

 having a thickness of 40 feet. The other two beds are much 

 shorter, the first below being not much over a half mile in exposed 

 outcrop, while the third rests directly on the diabase in the canon 

 leading up to the dairy. The middle one is about 1,000 feet below 

 the upper, and the third nearly the same distance below the middle. 

 The middle one outcrops prominently on the first ridge east of the 

 Lion's Head, while the third shows a thickness of fully 30 feet 

 where it has been cut by the road below the dairy. 



PetrograpJiy. — As a usual thing the rock is somewhat porous, a 

 condition due in large part to the thickly disseminated fragments 

 closely resembling pumice. Some portions of the rock are ex- 

 ceedingly compact and almost flinty in character. Scattered parti- 

 cles of a green serpentinous nature are also abundant in places, and 

 give a greenish hue to some outcrops. 



Thin sections prepared from the more massive varieties showed 

 a cloudy base, in which are imbedded crystals of feldspar and irreg- 

 ularly shaped isotropic areas which are water clear with the analyzer 

 removed. They are bounded by concave circular or elliptical 

 faces. The meeting points of these faces often appear drawn out in 

 long, slender rods, while still others show a cloudy circular area 

 within. The nature of these fragments is explained not only by 

 the optical behavior, but by the fact that in many cases they con- 

 tain crystals of feldspar partly within or attached on one side. The 

 cloudy base seems in large part to be made up of the same material, 

 only finely comminuted and undergoing alteration (Plate 2, Fig. 

 1). In one slide the glassy areas are observed undergoing devitre- 

 fication to a clear polarizing aggregate of minute grains which 

 resemble feldspar rather than quartz. That the isotropic fragments 

 are volcanic glass is certain, a glass which was exceedingly pumi- 

 ceous and frothy. The crystals of feldspar are not waterworn in the 

 slightest manner, appearing with sharp idiomorphic contours. 

 They are twinned on the carlsbad type, though in some cases poly- 

 synthetic twinning was noted. The extinction angles vary from 3-6 

 degrees, indicating an acid plagioclase. No traces of organic 

 remains appear in any portion of these volcanic ash beds, and with 



