62 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



There may hiive been periotls of elevation and subsidence siuce tbe ex- 

 istence of the hilies; if so, the disturbance has never been great. In 



some cases, where tlie river 

 ||^};}j'HjB=gm;r =^gs ^^ either in ancient or modern 



times, has washed the foot 

 of tlie mountains, the old 

 trachytes have been re- 

 moved, exposing the gran- 

 ites at the base. 



This basin is about fifty 

 miles in length, with an 

 average of five miles in 

 width. The lower thirty 

 miles present the most 

 "^ remarkable system of ter- 

 ■c races that I have'ever seen. 

 ^ There are usually three of 

 the terraces on cither side 

 ■3 of the Madison, and locally 

 ^ they may be increased to 

 six or eight. The great 

 f^ t feature in regard to them 

 "" > is their persistency and 

 •S uniformity, each terrace 

 £ being at the same eleva- 

 & tion as the corresponding 

 one on both sides. This is 

 c quite unusual. Generally, 

 I the terraces will be well 

 o displayed on one side of a 

 fi stream and scarcely seen 

 ,j. on the opposite side ; and, 

 S if they are shown on both 

 sides, there is not neces- 

 ^ sarily the same elevation 

 5 to corresponding terraces. 

 S The lake must have been 

 drained through the Lower 

 CaQon very slowly. The 

 general elevation of these 

 terraces may be estimated, 

 above the bed of the river, 

 about as follows: First 

 terrace, 10 feet ; second 

 terrace, 30 feet : third ter- 

 race, 100 feet. Messrs. 

 Gannett and Brown made 

 quite an extensive series 

 of observations with the 

 barometer, and the results 

 will be given in a subse- 

 quent portion of this re- 

 ])ort. These terraces are 

 well shown in Fig. IG. 



An interesting feature 

 nuiy be observed in this 



