F. GEOGRAPHY. 259 



in many other parts of the world, these coal-fields are of prac- 

 tical value only at comparatively 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 inex- 

 haustible, and the mines that can be economically worked 

 are of great number. 



In the Uintah 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 structure of the Henry Mountains, of which enough 

 had been learned in the preceding season to warrant the be- 

 lief that they embodied a type of eruption hitherto unknow^n. 

 The attention given to them has been amply repaid by the 

 elucidation of the manner of their constitution. They are 

 volcanic; but their lavas, instead of finding vent at the sur- 

 face of the ground, and piling up conical mountains there- 

 upon in the usual manner, 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 liave been 

 formed if the lava had risen to the surface; but the material 

 of the hill was sandstone and shale, instead of hard volcanic 

 rock. Subsequent erosion has carried away more or less 

 completel}'- the arching strata, and laid bare many of the 

 volcanic masses. It has revealed also a system of reticulating 

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

 intersecting the sedimentary rocks. The lava masses, the 

 dikes, and those portions of shale and sandstone which have 

 been metamorphosed by contact with the molten rock, are 

 harder than the unaltered sedimentary strata which sur- 

 round them, and yield to the agents of erosion more slowly. 

 The wash of rain and streams by which the face of the sur- 

 rounding land has been degraded, has been i-esisted by these 

 hard cores, and in virtue of their obduracy we have the 

 Henry Mountains. The deposits of lava are not all in juxt* 



