1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 671 



The rock, it will be observed, is somewhat anomalous as regards 

 both structure and chemical composition. Tliere is apparently little 

 doubt but that the groundmass is in all cases a mixture of potash and 

 soda lime feldsi)ars, as the microscope showed to be the case in the: 

 badly decomi)osed but holocrystalline variety, and with the possible 

 addition of si^oradic nepheline. The most striking feature is, perhaps 

 the high i)crcentage of i)otash in basic rocks so rich in magnesia. Both 

 on structural and chemical grounds one would at first be inclined to 

 regard the rock as belonging to the leucite, nepheline, or melilire 

 bearing series, but most careful tests have so far failed to establish the 

 jnesence of either mineral, beyond a possible doubt. If the powdered 

 rock is freed from the iron magnesian silicates by means of the electro- 

 magnet, ihe residual white gianules yield crystals of sodium chloride 

 when treated with hydrochloric acid, and minute radiating- crystals of 

 gypsum wluMi treated with sulphuric acid. It is probable, however, 

 that tliese reactions are i)roduced by the presence of zeolitic alteration 

 pro(hicts which the microscope shows to exist, but the exact nature of 

 which can not be made out. 



Tlie underlying syenitic rock (Xos. 73108 and T.'JIGO, U.S.jST.M.) is 

 a gray to pinkish, finely to coarsely crystalline granular rock, consist- 

 ing essentially of orthoclase and abundant spangles of black mica 

 readily determinable by the unaided eye, while on closer inspection are 

 seen abundant small deep greenish needle-like crystals of pyroxene. 

 These last in forms not over one mm. broad by ten mm. in length. 



In the thin section the rock is holocrystalline granular, and the 

 fel(lsi)ars so opaipie and muddied that their optical i)roperties are<iuite 

 obscure. They resemble the orthoclases of the older syenitic and 

 granitic rocks. Occasionally plagioclases occur, but which in nearly 

 every instance have gone over into a very light, almost colorless 

 decomposition product, at times almost wholly without action on 

 polarized light and recognizable as a })seudomorphous substance by, 

 their shar]) crystalline outlines. The mica occurs in broad (five mm.) 

 patches made up of a large number of independent folia, none of which 

 show hexagonal outlines, and also in long (five to ten mm.) spangles 

 radiating in every direction. Under the microscope it is deep smoky 

 brown in color, strongly dichroic, and shows extinction angles measured 

 against the cleavage lines in cross sections as high as 8°. The folia 

 are olten crushed, bent, and distorted, and show between the plates 

 inclosures of a finely granular colorless mineral aggregate, the nature of 

 which can not be made out. 



The augitic mineral occurs in beautifully perfect elongated forms, 

 sometimes as much as ten mm. in length, as above noted. In the section 

 it is only faintly greenish in color, not perceptibly pleochroic and gives 

 extinction angles, c on c, as high as 41°. Although the prismatic faces 

 are well developed the terminations, so far as observed, are never per- 

 fect but often jagged and full of inclosures or even broken into several 

 disconnected pieces which, though extinguishing simultaneously, are 



