114 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



covered by it for some distance. Its presence is so marked a feature in 

 tliis region that it cannot be overlooked. With regard to its age, I con- 

 sider the period of deposition as synchronous with that of the younger 

 portions of the Pliocene marls and shales. It is found near the edges of 

 the ancient lake, and was probably carried there by the waters draining 

 into the former. It must not be mistaken for the glacial drift which 

 occurs in the same region. The relative positions alone of these two 

 deposits will easily determiue their character. 



EEUPTIVES. 



1^0 eruptives whatever are found in the area of which this chapter 

 treats, except those basaltic dikes which are inclosed by Prozoic granites. 

 Their general strike is northeast and southwest. In width they vary 



: from a few feet to several hundred. This occurrence, both in the Granite 

 Hills and in the granitic outcrops west of the Sweetwater Hills, presents 

 an instance of very old eruptives. Carboniferous and younger strata 

 were observed in direct contact with the granites as well as with the dikes, 

 but in no instance did the latter penetrate farther than the granites. In 

 Scotland a number of basaltic outcrops occur which are regarded as be- 



■ ing of Carboniferous age. Here we have an instance, however, of still 



. greater age. From a distance the dikes appear as prominent black bands, 

 generally closely conforming with the surface of the granites, but some- 

 times rising above it as the adjoining rocks have been removed by ero- 



. sion and disintegration. 



POST-TERTIARY EROSION. 



Since the cessation of stratified deposition a large amount of erosion 

 has shaped the present form of the Sweetwater region. Toward the 

 north great masses of material have been carried oft' in a northeasterly 

 direction, leaving the steep wall of the Sweetwater Plateau. The cen- 

 tral portion of the Tertiary groups has largely been cut away in a 

 direction of west to east. This has produced the " trough " in which 

 we now find the river. From the southward slope of the plateau-beds, 

 at least several hundred feet in thickness have been removed from over a 

 considerable area. Isolated, table-topped buttes, horizontally stratified, 

 indicate the former vertical extent of the Pliocene beds. Gulches, 

 ; ravines, and valleys have been carved into the easUy-eroded material, 

 and large portions of this have been transported eastward. At a rough 

 estimate, we may say that fully one-fourth of the Pliocene beds north of 

 the river has been carried away. Toward the south we notice exten- 

 . sive erosion also. It is most prominent near the Avestern termination of the 

 Tertiary groups. Hundreds of feet in thickness have here been cut away, 

 until the active waters found some stratum which arrested their work of 

 ' destruction. Long, narrow ridges are produced by a former system of 

 ample parallel drainage. Deep gulches, now dry, then contaiued rush- 

 ing streams of water. From the condition in which we find many of 

 these ancient water-courses, we may infer that the influx of large streams 

 ceased within a compara,tively short space of time after the period at 

 which they had attained their maximum proportions. The old beds show 

 a steep fall and but little accumulation of drift. Both of these charac- 

 teristics would be reversed had the quantities of water diminished at a 

 very slow ratio. 



From the slopes of the Sweetwater and Seminole Hills, as weU as 

 from that of the plateau, large quantities of drift were carried toward 



