WHITE.] ERUPTIVE KOCKS. 39 



hills of the Park Eauge. The relation which this group has to similar 

 gioups in different parts of that great western region remains for future 

 investigation to determine. Its relation to those immediately associated 

 with it is a matter of great interest, which will be much enhanced by any 

 paleontological testimony that its strata may yet furnish. 



In this district the Uinta Group occupies nearly the whole surface of 

 the western portion of Axial Basin, comparatively small areas immedi- 

 ately east and immediately north of Yampa Mountain, and a consider- 

 able portion of the space between Junction Mountain and the eastern 

 end of the Uinta Uplift, all of which spaces are in unbroken continuity. 

 It also occupies quite a large space in the western part of the district, 

 which is bordered, in a general way, by Kaven Eidge, Eed Bluff Wash, 

 and the western boundary-line of the district, beyond which it is con- 

 tinued far to the westward. Throughout the whole of this last-men- 

 tioned space the strata of the group have tbat dull rusty-red aspect and 

 partial bad-land character before mentioned, while to the eastward of 

 the Uinta Eange the general aspect of the group is gray. 



ERUPTIVE EOCKS. 



Besides the stratified rocks of this district, which have already been 

 described, a few of igneous origin are found in the eastern portion 

 These are remaining portionsof a most remarkable outflow of dark colored 

 vesicular bassaltic trap, which, alter the close of the Tertiary period, 

 took place in the region bordering upon the eastern portion of this dis- 

 trict, extending also to the northward and southward. Much the larger 

 part of this great outflow has evidently been removed by erosion so that 

 only shreds of it remain where it once occupied the surface as a contin- 

 uous sheet. Broken masses of this rock are scattered [)rofusely upon 

 nearly all the higher hills in the northeastern portion of the district, 

 but upon only one point within its borders was it seen to occupy its 

 original undisturbed position. Just beyond the border of the district to 

 the eastward, however, especially u[)on the White Eiver Plateau, it yet 

 abundantly overspreads the stintified rocks upon which it was deposited 

 at the time of the outflow. This trap-outflow is known to have taken 

 place after the deposition of the Uinta Group because the trap is found 

 to rest upon the latter at Fortification Butte, some six miles north of 

 the northeast corner of this district. It is known to be of older date 

 than the drift of that region, and also older than other Quaternary 

 changes that have taken place there, because it, together with other 

 rocks, has suffered such extensive erosion since its deposition, and 

 because the drift-pebbles are in part composed of the trap. 



Other outflows of similar rock have occurred at other localities in the 

 Park Eange, some of which were probably contemporaneous with this 

 one, and it is probable also that this one was produced from more than 

 one vent. The only vents I was able to observe, however, was one in the 

 form of a dike, some eighteen miles north of the northeast corner of 

 this district, and another at "Chimney Eock" in Egeria Park, in the 

 mountains east of the district. This dike consists of a vertical wall about 

 twenty feet wide and three or four miles long. The rock of the dikes is 

 similar to that of the outspread trap, but is much less vesicular. Indeed, 

 it was not often that any vesicles could be detected by the unassisted eye 

 in the rock of the first-named dike, and these were flattened in the ver- 

 tical plane of the dike itself. It seems to have been only in the hori- 

 zontal portion of the outflow, where the pressure was at its minimum, that 

 the vesicular character of the rock became most marked ; and in some of 

 the layers, for it sometimes has an indistinct appearance of stratification, 

 the vesicular character is wanting. 



