W. Cross — Igneous Rocks in Wyoming. 127 



In view of the somewhat porous texture, the fluidal struc- 

 ture, and the presence of a glassy base in the rock of Pilot 

 Butte, it is most plausible to regard the mass as a remnant of 

 a surface flow. 



Description. 



Megascopical appearance. — In general the megascopical 

 descriptions of the Pilot Butte rock given by Emmons and 

 Kemp in their cited publications apply to my own material. 

 The rock is ashen -gray, yellowish, or greenish, and generally 

 porous in a subordinate degree. As the pores of the gray rock 

 are almost free from secondary minerals, while those of the 

 yellowish variety contain white minerals of zeolitic character, 

 the former seems probably the normal color of the fresh rock. 

 The pores are small, very irregular in form, and are but slightly 

 drawn out in any direction. The fractured faces are quite 

 rough and uneven. 



To the unaided eye the rock is largely dull and felsitic, with 

 numerous small reddish specks showing strong cleavage. 

 These are somewhat less than l mm in diameter, and all belong 

 to phlogopite. The uniform distribution of these mica grains 

 is very noticeable in all specimens I have seen. Occasional 

 smooth surfaces, when examined by a hand lens, show minute 

 pale-green prisms in a network. These are doubtless diopside, 

 the principal constituent of the rock. 



Constitution. — On microscopical examination it appears that 

 colorless diopside, phlogopite, and probable perofskite, are the 

 chief minerals of this rock, with a glassy base of brownish 

 color, which according to the analysis must contain silica, 

 alumina, and the alkalies in nearly the proper ratio to have 

 caused the formation of leucite had the mass entirely crystal- 

 lized. 



The development of diopside in this rock is very similar to 

 that described for the same mineral in the wyomingite of the 

 Boar's Tusk. It appears in colorless, doubly terminated prisms 

 less than l ram in average length, more or less markedly arranged 

 in streams. Cross sections show common development of the 

 prism and orthopinacoid, and occasional twinning parallel to 

 this latter plane. An analysis of diopside is to be given 

 later on. 



The next most important mineral is mica, which is very 

 similar in many respects to that in the rocks of the Leucite 

 Hills, but exhibits certain peculiarities of development worthy 

 of notice. Instead of being well crystallized and free from 

 inclusions, as in the other types, the phlogopite of the Pilot 

 Butte rock occurs in roundish grains averaging 0*8 mm in diam- 



