, — *. hoe oo Pe 7 , >.” | SON Vea eee FON at ee 4 RL’ Gee eS ge a ae ee ee 
: = =e t : wet hoe at ¢ rl +. 
7 9 ce ’ . >*S WJ 16 
72 B. N. A. BOUNDARY COMMISSION. 

157. The great series of limestones attributed to the Carboniferous 
period, are also found to repose unconformably on the rocks beneath | 4 
them. They lie in some places directly on the gneissic rocks, whilein 
other localities, 1,500 feet to 2,000 feet of Silurian strata intervene.* The ih 
entire thickness of the Carboniferous rocks is estimated at from 1,000 to 
2,000 feet. + 
158. Other limestones, supposed to be Lower Silurian, are also de- 
scribed in this region, and said to have a “much older look than those of 
the Carboniferous.” They are also “more compact, and contain a greater 
per centage of silica, are full of cavities lined with crystals of quartz, and 
weather into much more rugged forms.” ¢ In another locality, limestones 
belonging to the two series are found almost in contact, and “ although 
there is no positive non-conformity between the Carboniferous and 
Silurian, yet there appears to be a well defined physical line of separation. 
The Silurian limestones are more massive, brittle, cherty, and have an 
ancient look, while the Carboniferous beds are more pure limestone, and 
e. with thin, well defined layers.” § 

i59. On the Gallatin River the thickness of the various formations 
is estimated as follows :—Jurassic, 1,200 feet ; Carboniferous, 2,500 to 
3,000 ; Silurian, 800 to 1,000. || Near the junction of the Gallatin and 
Missouri, a section shows “ about 2,000 feet of metamorphosed slates, 
clays, and quartzites of all textures and colours, but mostly thinly lamin- 
ated,” which Dr. Hayden has no doubt belong to the Potsdam group. 
Compact, grey limestones, also of Silurian age, are developed in this 
locality, and overlying the whole series the Carboniferous limestone is 
found. 4[ The general resemblance of this section to that represented by 
Series C. and D. on the Boundary, is at once apparent. 
ee ee ee ee eee ee ee 
_ sae; SS ee Sl 
160. At Jackass Creek, and near Gallatin City, sections more or less 
closely comparable with those on the Boundary-line are also met with,** | 
. but many of the beds entering into the composition of the mountains in 
Montana, seem to be somewhat local and variable in their appearance. 
161. In the mountains at the head waters of the Powder River, five 
hundred miles south-easterly from the Boundary sections, the Carbonifer- 
ous limestone, again occurs, as a compact, cherty, yellowish-white rock 
with few fossils, but enough to indicate its age. It there rests directly 
on an irregular surface of quartzites and sandstones attributed to the 
*U. 8. Geol. Surv. Territ., 1872., p. 28. ‘ 
t Ibid., p. 29. | Ibid., p. 31, § Ibid., p. 78 || Ibid., p, 80. q Ibid., p. 69. ** Ibid., pp. 162, 172. 

