F. GEOGRAPHY. 289 



and the whole structure of the country is exhibited in its 

 reliefs. 



The eruptions of the Henry Mountains are of exceeding 

 interest. A description of a single one, though it will not 

 stand for all, will serve to illustrate the type. 



Mount Ellsworth is round, and its base is six or eight 

 miles broad. The strata of the plain about it are horizontal 

 on every side, except at one point. At its base the level 

 strata become slightly inclined, rising from all sides toward 

 the mountain. Near the mountain the dip steadily increases, 

 until on the steep flanks it reaches a maximum of forty-five 

 degrees. Then it begins to diminish, and the strata arch 

 over the crest in a complete dome. But the top of the dome 

 has cracked open, and tapering fissures have run out to the 

 flanks, and they have been filled with molten rock, which has 

 congealed and formed dikes. Moreover, the curving strata 

 of sandstone and shale have in places cleaved apart, and ad- 

 mitted sheets of lava between them. So the mountain is a 

 dome or bubble of sedimentary rocks, with an eruptive core, 

 with a system of radial dikes, and with a system of dikes in- 

 terleaved with the strata ; it is a mountain of uplifted strata, 

 distended and suffused by eruptive rock. 



The stratified rocks examined range in age from Carbon- 

 iferous to Tertiary. Upon the geological map the combined 

 Trias and Jura will cover half the space ; the volcanic area 

 will come next in size, and after that the Cretaceous. The 

 excellence of the topographic work will enable a very thor- 

 ough delineation of their boundaries. Fossils w T ere found in 

 numerous localities, but no large collection was made. Coal 

 of Cretaceous Age was seen in abundance, but no other valu- 

 able mineral. 



At the request of Major Powell, Captain C. E. Dutton, of 

 the Ordnance Corps, was directed by the Secretary of War 

 to accompany the former to his field of survey, and was en- 

 gaged in the examination of a large tract consisting of igne- 

 ous rocks. It lies in Southern Utah, its northern boundary 

 being about 150 miles south of Salt Lake City, extending 

 thence southward about 85 miles, having a breadth of about 

 60 miles. It consists of a series of long, narrow tables with 

 intervening valleys, and is structurally a repetition of those 

 features described by Major Powell as characteristic of the 



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