7f) APPENDIX TO REPORT OF THE SECRETARY. 



face-waters, so that the rain and melting snows are gathered more 

 quickly and thoroughly into rills and streams ; and both these influences 

 increase the inflow of the lake. 



This discussion has an important bearing upon the agriculture of the 

 arid region, for if tlie theory favored by these gentlemen is the true one 

 the work of irrigation can be pushed forward with the confident assur- 

 ance that the supply of water is more likely to increase than diminish 

 in the future; and it may even be jiossible, when the subject has been 

 fully developed, to devise measures which shall directly promote the 

 increase. 



Geological ivork by Mr. Gilbert. — During the preceding summer Mr. 

 Gilbert had discovered a peculiar series of phenomena produced by re- 

 cent orographic displacements, and he has this year found opportunity 

 to study them in numerous new localities. It appears that the system 

 of faults and flexures, the system of upward and downward movements, 

 by which the mountain-ranges and the valleys of Utah and Nevada 

 were produced, have continued down to the present time. Evidence of 

 recent movement has been discovered on the lines of many ancient 

 faults. The ancient shore-line of Great Salt Lake, which is exhibited 

 so conspicuously upon the surrounding mountain-slopes, and which must 

 originally have been level, is no longer so, but has been shifted up and 

 down by the displacement of the mountains. Its present altitude above 

 Great Salt Lake M'as determined at four different points by spirit-level, 

 and the determinations were found to range from 966 feet to 1,059 

 feet. The measurements by level were all made in the immediate vicin- 

 ity of the lake, but the barometer indicates that at points more remote 

 the discrepancy is several times greater. 



These observations are valuable additions to onr evidence that mount- 

 ain-making is a work of the present as well as of past ages, and that 

 the grand displacements by faults and folds are caused by slow and 

 intermittent movements. 



Mr. Gilbert also traced and mapped the northern portion of the 

 ancient shore-line of the lake from Salt Lake City to Kedding Spring, 

 following its sinuous course for 900 miles in Utah, Idaho, and Nevada, 

 and demonstrating that the ancient outlet he had discovered the pre- 

 ceding summer at Ked Rock Pass was the only one by which the lake 

 had ever discharged its water to the Snake Kiver. 



During the winter of 1876-'77, Mr. Gilbert prepared his report on the 

 geology of the Henry Mountains, and the manuscript was sent to the 

 printer. The Henry Mountains constitute a small group in southeastern 

 Utah and stand quite by themselves. They are of a i)eculiar character 

 and represent a type of structure that has never before been fully de- 

 scribed. Mr. Gilbert's report is a monograph at once of the mountain 

 group and of the type of mountain structure. The mountains are of 

 igneous origin, but the rising lavas, instead of outpouring at the surface 

 of the earth in the usual way, failed to penetrate the upper portions of 



