about the head of the Missouri, &c. 233 
observed them have the same igneous origin and the mountains 
l along the sources of the different branches of the Columbia 
exhibit these rocks in their full force. In Pierre’s Hole, Jack- 
these ancient volcanic rocks seems to have been poured out 
over the country and to have cooled in layers, giving to vast 
thicknesses of the rocks the appearance of stratified beds. 
€ mountains about the sources of the Missouri and Yellow 
Stone rivers are of eruptive origin and in the valley of the 
Madison fork of the Missouri are vertical walls of these ancient 
voleanic rocks one thousand to fifteen hundred feet in height, 
exhibiting the appearance of regularly stratified deposits dip- 
ping at a considerable angle. As we pass down tlr¢ Madison we 
find some beds of feldspathic rocks and mica and clay slates 
beneath the eruptive layers, dipping at the same angle. A 
passing the divide below the three forks of the Missouri we see a 
number of partially detached ranges which appear to be of the 
fame igneous character. In the Belt, Highwood mountains and 
Indeed all along the eastern slope in this region we find continual 
evidence of the outpouring of the fluid material in the form of 
surface beds or in layers thrust between the fossiliferous strata. 
hese igneous beds thin out rapidly as we recede from the point 
of effusion. A large number of these centres of protrusion 
may be seen along the slope of the mountains west of the Judith 
range. The erupted material sometimes presents a vertical wall 
three hundred feet high, then suddenly thins out and disappears. 
The Judith, Bear’s Paw and Little Rocky Mountains seem to be 
omposed for the most part of granite and other rocks, with 
paper 
et observations have convinced me that these rocks which I 
Reds defined by the term eruptive compose by far the greater 
h st. 
The a II. Potspam Sanpsrone. (Silurian.) = 
ene discovery of this formation in its western extension has 
ad y been announced in a former paper.* It was first made 
OWN as occurring in the Black Hills and resting upon the up- 
turned or nearly vertical edges of the schists, clay slates and 
Btanito nitoid rocks, and the inference was drawn that the same rocks 
‘# This Jour., [2], xxvi, 276. 
