100 



GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



long and half a mile wide. The deposit now can be seen far out in the 

 deeper portions of the lake, and the bubbles that arise to the surface in 

 various places indicate the x^resence at the orifice of a hot spring 

 beneath. Some of the funnel-shaped craters extend out so far into the 

 lake that the members of our party stood upon the silicious mound, 

 extended the rod into the deeper waters, and caught the trout and 

 cooked them in the boiling spring without removing them from the 

 hook. These orifices, or chimneys, have no connection with the waters 

 of the lake. The hot fumes coming up through fissures extending down 

 toward the interior of the earth are confined within the walls of the 



Fig. 



SECTION OF LARGE SPRING, YELLOWSTONE LAKE. 



walking over, it seems like treading 

 shells along the sea-shore. ' Much 



orifice, which are mostly circular 

 and beautifully lined with delicate 

 porcelain. Figure 34 exhibits a 

 fine cross-section of one of these 

 funnel-shaped basins. Wherever 

 the heated water issues from ori- 

 fices at the bottom of the lake the 

 temperature is changed. The 

 deposit of silica along the shore 

 has been built uj) in extremely 

 thin layers, or laminae, never more 

 than the sixteenth of an inch in 

 thickness. The shore, for several 

 yards in width, is covered to a 

 considerable thickness with the 

 disintegrated silica, so that in 

 on the broken fragments of washed 

 of the debris has been cemented 



Fig 



together, so that there are large masses scattered around, like the 

 Florida coquina. 



The question will arise as to the time that must have elapsed during 

 the deposition of this thick bed of silica. We may take the position 

 that no new groups of springs break out, or have done so in modern 

 times. Isolated springs connected with groups may form new openings, 

 however. We may. therefore, start from the period of the cessation of 

 the volcanic forces 'of this region, and trace the history down to the 

 present time. Very numerous groups have gone through with their 

 period of activity, and now nothing but a mass of ruins is left. It is 

 quite possible that this group mani- 

 fested its greatest power when the 

 lake extended all over the belt. The 

 waters of the lake have undoubtedly 

 receded from the area occupied by 

 this belt of springs within a compara- 

 tively recent period. We may say that 

 the deposition of the beds, so far as 

 is shown by any evidence we can 

 gather at this time, has probably oc- 

 cupied one or two thousand years. 



The springs of this group are very 

 numerous, of great variety and inter- 

 est, but there are no true geysers. 

 Some of these are what I would call 

 pulsating springs ; that is, the water 

 rises and falls in the orifice with great reo'ularity once in two or three 

 seconds. There are also a great number of mud-springs high up on the 



MUD PUFF, YELLOWSTONE RIVER. 



