178 



J. F. KEMP — THE LEUCITE HILLS OF WYOMING. 



end of the south mesa, at locality number 7, as shown on the map (figure 

 1). The leucites are also the largest, and are illustrated in figure 2, 

 which is simply a reproduction of an inked-in photomicrograph .* There 

 is hardly any other mineral except the leucite and biotite in the slide. 

 A little apatite, a rare needle of augite, and a grain or two of magnetite 

 are the only ones recognizable. All the other slides from the Leucite 

 hills have more or less sanidine. 



VARIETIES OF ROCK WITH MORE OR LESS SANIDINE. 



From the variety rich in leucite referred to above a series may be estab- 

 lished through those with more and more sanidine, until at the extreme 

 leucite is quite rare. In the intermediate varieties the leucites tend to 



be aggregated in swarms of mi- 

 nute crystals, while the sanidine 

 makes up the chief portion of the 

 remainder of the rock. The san- 

 idine in ordinary light sometimes 

 resembles a clear colorless glass, 

 occurring as it does in the very 

 cellular varieties, but the mo- 

 ment the upper nicol is put on 

 its true character is evident. It 

 has an extinction on its long side 

 up to 8 or 10 degrees, and is occa- 

 sionally once- twinned, although 

 twinning is uncommon. The 

 sanidine contains a network of 

 minute augite needles which 

 cross one another in every direc- 

 tion, and when it is well crystal- 

 lized it has the form of rather 

 stout rectangular rods. 

 An isotropic mineral is also present in one or two of the slides and in 

 anhedra up to a millimeter in cross-section. This is undoubtedly hauyne, 



* Figures 2 and 3 have been prepared by a method which combines the fidelity of a photograph 

 with the distinctness of a drawing. Photomicrographs of rock sections are seldom of much 

 significance. A negative was taken immediately from the eyepiece by a vertical camera. A blue 

 print was made from this and inked in with water-proof ink. The blue color was then bleached 

 out with a soda or ammonia solution, the print was washed, and the tendency to turn brown was 

 neutralized by immersion in very dilute hydrochloric acid. In this way a line drawing, black on 

 white, is afforded suitable for photoengraving, and much more quickly, accurately, and easily 

 prepared than with a camera-lncida. 



Figure 3.— Augite Phenocryst with Rim of Biotite, 

 from Black Rock Butte. 



The actual field is 2.5 millimeters. The ground- 

 mass is a dense aggregate of sanidine, leucite, and 

 augite, but no attempt has been made to differen- 

 tiate them. The white spaces are holes, either 

 amygdaloidal or produced in grinding. 



