GEOLOGICAL CHARACTER AND PETROGRAPHY. 175 



which did not flow far from its source. They doubtless indicate the 

 location of the vents which supplied the earlier flows, as well. On the 

 large, south mesa there are six cones, four of which are in pairs, while 

 the 5 other two are single. On Orenda butte there are one large and two 

 or three smaller cones. Black Rock butte shows no cone, and Pilot butte 

 has but a slight eminence or blister at its southern end. 



These volcanic hills are therefore intermediate in type between the 

 flat mesas such as the Table mountains, near Golden, Colorado, and the 

 plugs of the Black hills* They have the flat sheet protecting the un- 

 derlying sedimentaries, as in the former, and in the cones show the forms 

 assumed by viscous upwellings, not, however, confined in dikes or col- 

 umns nor brought out into relief, in the cases cited, by the removal of 

 surrounding walls of sedimentaries, as in the outlying igneous buttes 

 about the Black hills. 



The Leucite hills are surface flows. No dikes have been met about 

 them, nor are tuffs present in any case known to the writer. The rock 

 is often, if not almost always, of pronounced cellular character, produced 

 beyond question by expanding steam in a surface flow ; but even this 

 in the material collected by the writer hardly reaches the pumice stage. 

 The observed phenomena seem to have been produced by the outpouring 

 and lateral spread of a highly fluid lava in the early stages, followed in 

 the later ones by a similar viscous extrusion which remained quite near 

 the vent. 



Petrography. 



introductory. 



There is considerable variation in the rocks gathered at different parts 

 of the south mesa, and also between them and the others collected to 

 the north. Some of the former are extremely rich in leucite and cor- 

 respond to the rock described by Zirkel. Others, and increasingly so to 

 the north, are poor in this mineral, almost, if not quite, to its disappear- 

 ance, while orthoclase (sanidine) appears in larger and larger amounts. 

 Haiiyne is also present in relatively large anhedra, and, as later described, 

 fairly large phenocrysts of augite and olivine are found in material 

 from Black Rock butte. These phenocrysts are clearly visible to the 

 eye, and by the microscope are shown to be surrounded by rims of bio- 

 tite. Inclusions of sandstone and impure limestone are very abundant 

 and are met in all parts of the flows as studied. The rock from Pilot 

 butte is almost lacking in leucite, but while its biotite and augite are 



* I. C. Russell : On the Nature of Igneous Intrusions, Journal of Geology, vol. iv, pp. 23 and 176. 



