Allen.] 258 [January 21, 



Gros Ventres Fork of the Snake River. In describing this locality, 

 he says : " To-day the tertiary strata begin to assume a good deal of 

 importance. We have the brick-like materials which result from the 

 burning out of the lignite beds. There were also masses of indurated 

 clay, covered with vegetable remains and impure lignite beds ; in- 

 deed, all the indications which the lignite tertiary beds present on the 

 east side of the mountains." 1 



How far to the northward of the Missouri River these burned out 

 lignite beds extend, I have been unable to determine. That they do 

 not extend far in this direction seems probable, from the fact that no 

 mention is made of them in any reports of the surveys of the 49th 

 parallel ; neither does Hind refer to them in his report of the geology 

 of the Assinniboin and Saskatchewan Rivers. The valley of the 

 Musselshell, and the dividing ridge between the Musselshell and Yel- 

 lowstone, is apparently all cretaceous, and these metamorphosed beds 

 are hence limited to the vicinity of the Yellowstone River; neither do 

 they occur in the valley of the Yellowstone above the mouth of 

 Tongue River. As already noticed, their western limit, in the basin 

 of the Yellowstone, is the dividing ridge between the tributaries of the 

 Big Horn and Rosebud Rivers, while they extend southward along the 

 eastern base of the Big Horn Range to the sources of the Shyenne, 

 and thence eastward to the Black Hills and the valley of the Little 

 Missouri. They appear to occur not only along this river throughout 

 nearly its whole extent, but along all the tributaries of the Yellow- 

 stone east of the Big Horn. 2 



The time during which this peculiar metamorphism has been act- 

 ing extends back to a very remote period, this igneous material, in a 

 water-worn state, occurring in the drift that covers the general sur- 

 face of the country, often many miles from the nearest seat of meta- 

 morphic action, as well as in the terraces that border the larger 



1 Hayden's Rep. Geol. Expl. Missouri and Yellowstone, p. 86. 



J Pumice stone, it is well known, occurs in isolated, erratic fragments in Colo- 

 rado, near the base of the Rocky Mountains, and it hence becomes an interesting 

 question to ascertain whether it originated in the lignite region to the northward, 

 or in the volcanic districts of Montana, or whether the lignite beds in Colorado 

 have not at some points also been on fire, giving rise to these materials. The ques- 

 tion may perhaps also arise as to whether the volcanic fragments in the terraces of 

 the Yellowstone and its tributaries did not perhaps come from the volcanic district 

 to the westward. From the appearance of the materials, however — the baked 

 clays as well as the scoriaceous matter — I had not, while on the ground, any doubt 

 of their being formed by the burning out of the lignite beds of the immediate 

 vicinity in early post-tertiary times. 



