REPORT OF THE SECRETARY. 61 



secting 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 surround them, and yield to the agents of erosion more 

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

 rounding country has been degraded, has been resisted by these hard 

 cores, and in virtue of their obduracy we have the Henry Mountains. 

 The deposits of lava are not all in juxtaposition but are scattered in 

 clusters, and each cluster has created a mountain. Mount Ellen consists 

 of a score of individual lava masses ; Mount Pennell and Mount Hillers 

 of one principal mass, accompanied by several of minor importance; 

 Mount Holmes of two masses; Mount Ellsworth of a single one, with 

 many dikes and sheets. Each of the mountains is individual, topo- 

 graphically as well as structurally, and together they constitute a group 

 of mountains, not a range. Mr. Gilbert's note-books contain many 

 sketches, by the aid of which he will be able to illustrate all the features 

 of the peculiar types of structure. 



Before commencing the main work of the season, Mr. Gilbert made 

 an excursion in search of the outlet of Lake Bonneville, the great fossil 

 lake of Utah. During an epoch which was probably coincident with 

 the glacial epoch, the broad interior basin of Utah was covered by a 

 great lake which overflowed its rim and sent an outlet to the ocean by 

 nay of the Columbia River. When the climate became gradually 

 warmer and drier, the evaporation grew greater and the rain-fall grew 

 less, until finally the overflow ceased and the lake began to dry away 

 and shrink within its shores; to-day only Great Salt Lake and Seviei 

 Lake remain, but high up on the mountain is carved the Bonneville 

 Beach, a permanent record of the old flood-tide. The search for the 

 point of outlet was successful, and it was found at the north end of 

 Cache Valley, a few miles beyond the boundary of Utah, in the Terri- 

 tory of Idaho. The bed of the outflowing stream was traced for a num- 

 ber of miles. The beach-lines were seen to run quite to the pass through 

 which the channel was cut, but beyond, on the side of the drainage of 

 the Columbia, no trace of them could be seen. 



Of no less interest was the discovery of a recent orographic move- 

 ment at the western base of the Wasatch Range. A great fault runs 

 along that base ; one of the faults by which the mountains were pro- 

 duced. The block of the earth's crust which lies to the westward of the 

 fault-planes was dropped down, and the block which lies to the east- 

 ward was lifted up, and from the eastward block subsequent erosion 

 has carved the range. Along the plane of ancient movement there has 

 been a recent movement. The mountain has risen a little higher or the 

 valley floor has dropped a little lower, and this so recently that the 

 Bonneville flood is ancient in comparison. 



Capt. C. E. Duttou resumed this year his study of the large area 

 of igneous rocks in Southern Utah, in the vicinity of the Sevier River, 



