98 GEOLOGICAL SUEVEY OF THE TERRITOEIES. 



way that I can account for the appearance of these worms is, that the 

 fish swallows certain bugs or insects, and that the larvse formed from 

 them gradually develop into the full-grown intestinal worm. But even 

 if this explanation of their appearance was received, does it not seem a 

 little strange that while all the fish above the Upper Falls are more or 

 less affected by them, that below and even between the Upper and 

 Lower Falls such a thing as wormy trout is never heard of. Being 

 unable, with my limited knowledge of ichthyology, to arrive at any 

 definite conclusion in regard to their appearance, I submit the above 

 facts to those who are more learned than myself in this most interesting 

 branch of natural history." 



I will not, in this place, present a detailed description of this wonder- 

 ful lake, but simply notice it in general terms. As we proceed from 

 point to point around its borders, its most prominent features will be 

 described. We regard the lake-basin as due in iDart to erosion. All 

 along its margin are high banks and terraces, composed of a modern 

 stratified deposit, passing up into an aggregation of sand, pebbles, &c., 

 which is not unfrequently cemented into a tolerably firm conglomerate. 

 These deposits, which are made np of eroded volcanic rocks, have in 

 some instances the white appearance and somewhat the composition of 

 Pliocene clays, marls, and sands of the other lake-basins along the Mis- 

 souri and the Lower Yellowstone. In the northern portion of the basin, 

 these deposits reach a thickness of 300 to 600 feet, and must l^e of the 

 later Pliocene era and even extending down to the present time. The two 

 lakes were then connected, although probably never completely united. 

 The belt of mountains that separated them was about four miles in 

 width. I have estimated that, since the period of volcanic activity, the 

 depth of the lake has been about 500 feet greater than at present, the 

 shore-lines being then high upon the side of the surrounding moun- 

 tains. During the time of the greatest volcanic action, the waters 

 must have covered the loftiest peaks ; for many of them are composed 

 of the breccia or conglomerate in a regularly stratified condition. This 

 breccia surrounds the highest volcanic cones or nuclei, as Mounts 

 Doane, Stevenson, &c. The area occupied by the lake is now gradu- 

 ally but very slowly diminishing. Our course aYound the lake was 

 along the west side, from the outlet of the Yellowstone. Our i)urpose 

 was to make a careful topographical and geological survey of the shore-, 

 line, to note every bay or indentation, and every little stream that 

 poured its waters from the surrounding mountains. Messrs. Elliott 

 and Carrington made a careful topographical and pictorial chart of the 

 shore-lines as well as the islands from our boat, so that it is hardly 

 possible for the work to have been made more complete. The imme- 

 diate lake shores are paved with the volcanic rocks which form the rim 

 that surrounds it. Fragments of obsidian prevail, but there are great 

 quantities of the breccia and trachyte also. The immediate rim of the 

 basin on the west side is marked by a peculiar series of step-like 

 ridges, which are not continuous for long distances, but appear to be 

 the result of slides. The surface waters from the snows have doubt- 

 less gradually undermined vast portions of the mountain sides, and 

 they have fallen down at different levels, leaving between the detached 

 mass and the parent mountain a depressed interval of greater or less 

 width, in which there is a meadow-marsh or small lake. These steps 

 or terraces are covered with a dense growth of pines ; and even on 

 the sides of the mountains, which are so steep that it was impossi- 

 ble to ascend them with our animals, small groups of pines cling 

 to the thin soil. On account of the almost vertical sides of this 



