124 TT. Cross — Igneous Rocks in Wyoming. 



belonging to tbe rock. By close examination under a lens 

 small "glfstening cleavage surfaces may occasionally be seen, 

 which "are interrupted by many minute particles, produc- 

 ing a poikilitic structure. These surfaces belong to sanidine. 



The pores of the rock are always irregular in shape, ordi- 

 narily not drawn out in any prevalent direction, showing diver- 

 gent" smooth- walled arms. They vary from l cm in length 

 downward, and are also developed in variable amount. In 

 some places the cavities constitute half the bulk of the rock. 



With a hand lens of high power the walls of the cavities are 

 often seen to be coated by a network of very pale-yellowish 

 needles, and in rare cases they project free into the pores. 

 These needles are of the amphibole described below. A far 

 more common filling of the pores is hyalite, in its characteris- 

 tic clear globular forms. With the hyalite, and generally 

 embedded in it, is a white mineral in rude bundles, opaque, 

 and so poorly developed that it has not been determined. 



Mineral constitution and structure. — On microscopical study 

 it is found that this rock consists of leucite and sanidine in 

 predominant amount as compared with the ferro-magnesian- 

 lime elements, phlogopite, amphibole and diopside. Apatite 

 and rutile (?) are accessory minerals, but no magnetite, ilmenite 

 or pyrite occurs. Biotite is developed, as in the wyomingite, 

 in a few much resorbed flakes. It is probable, from examina- 

 tion of the chemical analyses, that free silica in the form of 

 tridymite or opal is present, but aside from the filling of cavi- 

 ties spoken of above neither substance has been identified. 



In quantitative development leucite and sanidine vary con- 

 siderably, now the one, now the other seeming to predominate, 

 but in general they are nearly equal in amount. Of the 

 heavier silicates phlogopite is the most important, while the 

 other two seem to vary with the leucite and sanidine. The 

 amphibole is developed approximately in proportion to the 

 sanidine, and diopside corresponds to leucite. No amphibole 

 has been found in the sanidine-free wyomingite. 



The peculiar association of minerals in these rocks leads to 

 several interesting microstructures. Phlogopite appears to 

 have formed first and is almost wholly free from inclusions. 

 Leucite and sanidine are as a rule grouped in separate patches 

 or areas, the former in swarms of minute anhedra exactly like 

 those of the wyomingite. Sanidine occurs in aggregates of 

 stout, square prisms, much larger than the leucites, but still 

 seldom exceeding l mm in length. 



Diopside is mainly developed in minute needles and micro- 

 lites, a large share of which are included in the sanidines, pro- 

 ducing a poikilitic structure which may occasionally be de- 

 tected megascopically. The remainder of the diopside occurs 



