60 REPORT OF THE SECRETARY. 



desert plains. The practical importance of this classification, if carefully 

 made, is great, not only in presenting the information desirable to those 

 who wish to settle in the country, but also in the collection of facts 

 neccessary to intelligent legislation concerning these lands. 



In the region embraced in this survey a very small portion of the 

 country can be redeemed by irrigation for agriculture, and no part of it 

 can be cultivated without irrigation. It appears from the reports that 

 less than one per cent, can be thus made available. Especial care has 

 been given to the determination of the extent of such lands, so as to 

 exhibit their position on the maps. These irrigable lands and timber 

 lands, together witli some small districts of coal-bearing lauds, are the 

 only parts of the country that should be surveyed into townships and sec- 

 tions. 



Having in view economy and convenience in the linear surveys of this 

 district, the geodetic points of the general geographic survey under the 

 direction of Professor Powell have been carefully marked, that they 

 may hereafter be used as datum points by the officers of the General 

 Land Office. 



Extensive coal-fields exist in the region surveyed, but, as in many 

 other parts of the world, these coal-fields are of practical value at com- 

 paratively few places. The general characteristics of these coal-fields 

 have been the subject of much investigation, and some very interesting 

 and valuable results have been reached ; these will appear in the final 

 reports. The quantity of available coal is practically inexhaustible, 

 and the mines that can be economically worked are of great number. 



In the Uinta Mountains silver and copper mines have been discovered 

 and worked by private parties. The extent of these silver and copper 

 bearing rocks has been determined, but their value can be established 

 only by extensive working. 



Mr. G. K. Gilbert devoted much of his time to the study of the struc- 

 ture of the Henry Mountains, of which enough had been learned in the 

 preceding season to warrant the belief that they embodied a type of 

 eruption hitherto unknown. The attention given to them has been am- 

 ply repaid by the elucidation of the manner of their constitution. They 

 are volcanic, but their lavas, instead of finding vent at the surface of 

 the ground and piling up conical mountains thereupon in the usual man- 

 ner, ceased to rise while still several thousands of feet underground, and 

 lifted the superincumbent strata, so as to make for themselves deep- 

 seated subterranean reservoirs, within which they congealed. Over 

 each of these reservoirs the strata were arched and a hill or mountain 

 was lifted equal in magnitude to that which would have been formed if 

 the lava had risen to the surface ; but the material of the hill was sand- 

 stone and shale instead of hard volcanic rock. Subsequent erosion has 

 carried away more or less completely the arching strata and laid bare 

 many of the intrusive masses. It has revealed also a system of reticu- 

 lating dikes which go forth in all directions from the main masses, inter- 



