F. GEOGRAPHY. 28 



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Provo, on the Utah Southern Railroad, and a secondary base- 

 station at Camp Supply, on the Dirty Devil River. 



Plane-table methods were used in the topographic work. 

 The plane table devised by Professor Thompson for this 

 special work, after three years' experience, proves to be sat- 

 isfactory,' and is believed to greatly increase the accuracy of 

 the work over the earlier methods of sketching and descrip- 

 tive field -notes. Free-hand sketches and profile sketches 

 were used as accessory methods in delineating the topo- 

 graphic features. 



The classification of the lands begun in former years was 

 continued during the past year; and it was found that of the 

 lands surveyed during the past season, one fourth of one per 

 cent, belongs to the first class, i. e,, lands which can be re- 

 deemed by irrigation ; about fifty per cent, to the second 

 class, i.e., pasture lands; about nine per cent, to the third 

 class, i. e., timber lands ; four per cent, to the fourth class, 

 i. e., mineral lands; and the remainder to the fifth class, i. e., 

 desert lands. 



Geology. Mr. G. K. Gilbert accompanied Professor Thomp- 

 son's party as geologist. His prime subject of study was 

 structural geology ; that is, the magnitude and character- 

 istics of the displacements by which rock-beds, originally 

 level, have been brought to their present uneven condition. 

 This study also involved an examination of the succession 

 of strata, and incidentally full material for a geological map 

 has been accumulated. A second and closely allied subject 

 of study has been the eruptions that produced the Henry 

 Mountains ; a third has been the erosion by which the 

 structure has been laid bare, and a fourth the Salina Creek 

 unconformity. 



The investigation at Salina Creek was of a special nature, 

 and its bearings can not be briefly stated. Its result estab- 

 lishes a single point of geological history : namely, that an 

 epoch of mountain growth, of which evidences are found in 

 the Sevier and San Pete valleys, and in the Pah-van Mount- 

 ains, occurred about the end of the Cretaceous period. 



A line drawn from the Mu-si-ni-a Plateau southward to 

 the eastern margin of the Aquarius Plateau separates a re- 

 gion of faults at the west from a region of folds at the east. 

 Faults and folds are not distinct types of displacement, but 



