118 ON THE GEOLOGY AND NATURAL HISTORY 
It is from these mountains that the numerous branches of the Platte have their sources, 
extending a distance of nearly four hundred miles. Again, the dynamical structure of 
this range warrants the inference that these rocks, in a more or less horizontal position, 
were continuous over the whole area at present occupied by it some time during the Tertiary 
period. 
The minerals of economical value are quite extensive. The gold mines of Pike’s Peak 
are in this range, and the evidence is quite conclusive that this precious metal will be 
found to a greater or less extent all along the eastern slope. Iron ore is found in great 
abundance and of the best quality, while along the North Platte and in the vicinity of 
Pike’s Peak, lignite is known to exist in considerable quantities, and has been used for fuel. 
The most important outlier of the Rocky mountains, on the eastern slope, is the Big 
Horn range, which, though somewhat irregular in the shape of its mass, has a general 
trend nearly northwest and southeast. It occupies an area about 180 miles in length and 
50 in breadth. Near latitude 432° and longitude 102° the line of fracture seems to have 
ceased toward the south or southeast and to have made a gradual flexure around to the 
west, the whole range soon losing its granitoid character and becoming entirely composed 
of more modern eruptive rocks. ‘The eruptive portion continues westward until it joins on 
to the Wind river range near the sources of Wind river. At the southern end of the Big 
Horn mountains, we can trace a single low anticlinal across the prairie, connecting these 
mountains with the Laramie range at Red buttes on the North Platte. We also know by 
the position of the fossiliferous strata upheaved along the mountains, that the gradual flex- 
ure of the range to the west joins on to the eruptive portion, and thus forms a connection 
with the Wind river range. 
The central portion of these mountains is also composed of granite and granitoid rocks, 
with the same series of fossiliferous formations, inclining at various angles from each side 
of the axis of elevation, as are seen around the Black hills and along the Laramie moun- 
tains. Some of the more lofty peaks are from 8000 to 12,000 feet above the sea, and are 
clothed with perpetual snow. We think that the evidence is quite clear that up to the 
time of the accumulation of a large portion of the Lignite Tertiary beds, all these 
formations, from the Silurian to the true Lignite strata inclusive, were in a horizontal posi- 
tion, extending continuously over the whole area occupied by the mountains, but as they 
were slowly elevated, the central portions were removed by the erosive action of atmo- 
spheric agencies. The eruptive portion which unites the Big Horn range with the Wind 
river mountains is exceedingly picturesque, presenting the appearance of a connected series 
of basaltic cones, and so rugged and inaccessible are they that the persevering trappers 
haye never been able to penetrate them in their hunting explorations. 
Like the Black hills, the Big Horm range does not give rise to many important sub- 
