NATURE OF ERUPTIVE MESAS 335 



and it is in great contrast in this respect to any other eruptive known to 

 the writers, unless it be certain minettes. 



In the dikes associated with the volcanic necks the rock is naturally 

 more compact than in the surface flows, but in the tAVO exposures the 

 Boars tusk and the Badgers teeth it is not entirely devoid of amygdules. 



Do the different flows difl'er in chemical and mineralogical composi- 

 tion ? That there is considerable variation within narrow limits among 

 the rocks of the several mesas, buttes, and necks both Doctor Cross and 

 one of us have earlier shown. All the variations are due to differing rela- 

 tive amounts of practically the same ingredients, namely, leucite, sani- 

 dine, diopside, phlogopite, a rare amphibole, rarely aegirite, apatite, and 

 unindividualized base. The last named, of course, introduces marked 

 contrasts in texture. Diopside, phlogopite, and apatite are common to all 

 the exposures, but any one of the others may entirely fail. No attention 

 has been previously given to the question as to whether this variation, 

 if occurring in the same mesa, is present in the same or difi'erent flows. 

 In our collecting we have endeavored to give emphasis to this feature, 

 and have concluded that there is variation in the same flow. 



Secondary Minerals 



Very rarely secondary minerals have formed in the amygdules. As 

 a rule, they are empty. Doctor Cross has reported a potash niter, and 

 we have found chalcedony at Steamboat mesa and aragonite at the 

 Badgers teeth. On the surface of North Pilot there are numerous small 

 concretions and crusts of a mineral rich in phosphoric acid containing 

 calcium, magnesium, and aluminum of varying percentages. Thus far 

 it has been impossible to refer it to a mineral species. This same min- 

 eral is occasionally found in the cavities of the porous varieties of the 

 leucite rock. 



Another mineral deposit which is frequently to be noted around the 

 escarpment is a crust produced by mountain rats. It is now being in- 

 vestigated. 



Sand Glaciers and Dunes 



To the southwest of Steamboat mesa a ridge runs away, leaving a pass 

 between the two. Over this pass great volumes of white sand are con- 

 stantly being blown from the regions to the west by the prevailing winds, 

 which are westerly. The sand sweeps up over the crest from a great 

 stretch of barren dunes running toward Killpacker creek and rolls down 



