62 



GEOLOGICAL SURVEY OF THE TERRITOEIES. 



There may have been periods of elevation and subsidence since the ex- 

 istence of the hikes | if so, the disturl)ance has never been great. In 



some cases, where the river 



either in ancient or modern 

 times, has washed the foot 

 of the 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 live miles in 



width. The lower thirty 



_ miles present the most 



'^ remarkable system of ter- 



■c races that I have ever seen. 



I There are usually three of 



■^ the terraces on either side 



I of the Madison, and locally 



^ they may be increased to 



I six or eight. The great 



^ feature in regard to them 



> is their persistency and 



ri uniformity, each terrace 



5 being at the same eleva- 



6 tion as the corresponding 

 i one on both sides. This is 

 c quite unusual. Generally, 

 ^ the terraces will be well 

 I displayed on one side of a 

 S stream and scarcely seen 

 ^ on the opposite side ; and, 

 S if they are shown on both 

 < sides, there is not neces- 

 H sarily the same elevation 

 S to corresponding terraces. 



The lake must have been 

 ■-, drained through the Lower 

 J Canon very slowly. The 

 ^ general elevation of these 



1 terraces may be estimated, 

 i 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- 

 port. These terraces are 

 well shown in Fig. 16. 



An interesting feature 

 may be observed in this 



