PETKOGKAPHY OF ZIKKEL MESA 315 



representatives being considered, as they have but inconspicuous develop- 

 ment, to be small phlogopite crystals in the groundmass. In other 

 respects it does not vary essentialh^ from the typical wyomingite of 

 Cross. Doctor Cross gathered wyomingite near the Fifteen-mile spring, 

 but although we have prepared several slides from material collected 

 there b,y us, the}^ all proved to be orendite. There is little doubt that 

 w3'omingite is merely a local development in a flow which is elsewhere 

 chiefly orendite. Chemically, as shown by Cross, the rocks are practi- 

 call}^ identical, since of analyses IV and V given in his paper, IV being 

 wyomingite and V orendite, no two percentages of the same oxide diff'er 

 more than two or three tenths of one per cent, except the alumina. 

 This in wyomingite is about one and a half per cent more than in oren- 

 dite. The others hardly differ beyond the ordinar}^ variations of dupli- 

 cate analyses. If, however, in one rock we have a large percentage of 

 sanidine with 64 per cent of SiOj and in another leucite with only 55, of 

 necessity the excess left b}^ the latter must be present in some more acid 

 silicate or in a highly silicious glass. Doctor Cross's observations with 

 the microscope led him to believe that the latter existed in the slides. 



Even in the orendite one often remarks the tendency of the leucite to 

 gather in swarms in certain parts of the slides, and to thus afford patches 

 of W3^omingite in the midst of a general orendite, and the rock wyo- 

 mingite is only a development of this character in a mass at least large 

 enough to afi'ord a hand specimen, and it may be indefiniteh^ larger. 

 We can detect no mineralogical difi'erence between difi'erent flows. 

 Should future visitors to the Hills desire to collect WN^omingite, it will 

 be safer to do so from the dikes as subsequently described or from the 

 South Table. 



The orendite of Zirkel mesa corresponds in all essentials to the de- 

 scriptions of Doctor Cross. It is illustrated by the photomicrograph on 

 plate 38, figure 2. It consists of leucite, sanidine, diopside, variable 

 amounts of hornblende, phlogopite, apatite, and what is doubtless rutile. 

 The relative amounts of each of these var}^ considerabl}^ The horn- 

 blende may fail, but in careful search it is almost always to be found at 

 least in small amounts. In the latter case it is, in its usual section, so 

 near the color of the phlogopite that, as in the earlier observations of one 

 of us, it may escape notice. The ray, vibrating parallel with h, is often 

 of a violet hue, suggesting the characteristic colors of the titaniferous 

 pyroxenes. Doctor Cross mentions rutile with a query. There is little 

 doubt that this mineral is present and sometimes in considerable quan- 

 tity. It is a deep reddish brown, and is sometimes prismatic and again 

 in small irregular masses. In some of the other mesas it has been found 

 in characteristic sagenite nets in the phlogopite. 



