W. Cross — Igneous Rocks in Wyoming. 131 



Discussion of analyses. — It is interesting to note that the 

 older analyses, in spite of the evident inaccuracies, indicate 

 quite correctly the general character of the magmas of the 

 Leucite Hills ; but they fail to show the great complexity of 

 constitution which makes these rocks so noteworthy. Few 

 rocks have been shown to contain so many chemical elements 

 in determinable amounts; and in this connection it should be 

 stated that in all probability zirconia was present in all the 

 rocks, but was not tested for in the older analyses, III, IY, V, 

 and VII, which were made several years ago, while VI was 

 made in January, 1897. 



From the analyses of phlogopite and diopside it is plain that 

 Ti0 2 , Cr 2 3 , Ba6, and Fl are in very large degree contained in 

 the mica, while the first is the only one of these oxides in the 

 diopside. Tests showed that the sulphuric acid was always in 

 the part of the rock soluble in HC1, hence it is certain that 

 barite cannot be present, and the probability appears that 

 noselite is developed to a varying extent in minute crystals not 

 distinguished from leucite in the thin sections. There is an 

 unusually large amount of P 2 5 in all these rocks, and while a 

 few large apatites may be readily detected under the micro- 

 scope, it seems probable that much of this mineral is developed 

 with diopside in minute needles not easily recognized. The 

 presence of such a large amount of phosphoric acid here con- 

 trasts in a noteworthy manner with the very small amount 

 ordinarily found in phonolites. 



It is noteworthy that the rock of the volcanic neck (III) is 

 richer in almost all of the rarer constituents than any other 

 except that of Pilot Butte. It is also higher in magnesia and 

 lower in silica than the others. As for zirconia the suggestion 

 is made that the peculiar amphibole of orendite is perhaps zir- 

 conia-bearing, analogous to lavenite, wohlerite, and hiortdahlite. 

 If that is the case it seems quite possible that the amphibole 

 contains Ce 2 3 and Di 2 O s . 



The most striking fact of petrographical interest in these 

 analyses is the almost identical constitution of two rocks, one 

 rich in leucite and free from sanidine, the other with predomi- 

 nant sanidine. The conclusion that chemical composition of a 

 magma does not alone determine whether leucite or sanidine 

 shall be formed, but that this is controlled by conditions of con- 

 solidation is unavoidable. As the composition of the amphi- 

 bole in the orendite is unknown, a satisfactory calculation of 

 analyses Y and YI is impossible, but those of the other types 

 afford interesting results. 



Taking first the Boar's Tusk wyomingite, the molecular ratio 

 of its constituents is as below ; and on assuming that lime is 

 wholly in apatite and diopside, magnesia in diopside and phlo- 



