122 W. Cross— Igneous Rocks in Wyoming. 



acid found by analysis, these rocks are shown to be richer in 

 apatite than would be inferred from microscopical examination. 

 The apatite is seen occasionally in rather large grains, but must 

 be chiefly developed in small prisms difficult to distinguish 

 from diopside needles. 



By the close crowding together of the leucites and the occur- 

 rence of diopside needles between them, it would seem as if 

 there could be very little glassy base present ; but in places 

 there is a filmy globulitic substance between the leucites, and 

 as will appear from the discussion of the chemical analyses, 

 there are grounds for supposing that there must be a highly 

 siliceous residue in the form of a colorless glass forming a base 

 for the minute crystals. Careful reexamination of this rock, 

 after the difficulties in interpreting the analyses were fully 

 shown, convinces me that there is a much larger amount of 

 residual glass between the minute leucites and the felt of diop- 

 side and apatite needles than was at first suspected. The leu- 

 cites do not often interlock with angular projections, and 

 although the diopside needles fill in the gaps to a large degree, 

 there is some glass undoubtedly present, 



The dense wyomingite of the above character undoubtedly 

 grades with all intermediate stages into the rock containing 

 much sanidine, but several specimens were collected showing 

 no feldspar except in a somewhat questionable interstitial form. 

 Probably all thin sections exhibit a few minute leaves of dark- 

 brown biotite containing so much ferritic matter, seemingly a 

 product of magmatic resorption, as to obscure the optical pro- 

 perties. Kemp mentions a few grains of haiiynite : magnetite, 

 titanite, pyrite, and other accessory minerals seem entirely 

 wanting in the pure wyomingite. 



Both Zirkel and Kemp have represented the microstructure 

 of this rock, and their descriptions agree with the above in 

 most particulars. Chemical analyses of the wyomingite will 

 be given in a later section of this article, together with those 

 of the other types and a discussion of the systematic position 

 of all the rocks described. 



From the Boar's Tusk. — The specimens of massive rock 

 from this volcanic neck are much like the type already de- 

 scribed, with a slightly larger amount of phlogopite and a still 

 more pronounced schistose structure than was observed at any 

 point in the Leucite Hills proper. The groundmass is, as a 

 rule, of a more or less distinct dull-greenish shade, and there 

 are some flat, drawn-out vesicles, rarely containing any 

 secondary minerals. 



The breccia in the southern part of the neck is chiefly made 

 up of wyomingite fragments less than 2 inches in diameter, 

 and they seem to all belong to the same variety as the massive 



