32 BOTANICAL, INFORMATION'. 



of about 80°, but are, for the most part, quite vertical. By 

 following so close upon each other, these dykes give these 

 mountains a striped appearance. These ranges occur again 

 in the third group, and are separated by firm arenaceous 

 plains from the second. At the south-eastern confines, at 

 least on this route, occur high 



D. — Mountainous ridges, which on one side show escarp- 

 ments, fronting defiles and narrow river-valleys. These 

 fronts exhibit the first pseudo-columnar basalt, of an earthy 

 fracture, but with an ore-like hard surface. Above, are 

 strata of black iron-sandstone, very heavy, orelike, and 

 densely grained. This is, perhaps, the magnetic iron-stone, 

 mentioned by Captain Fremont as occurring at Sweet Water 

 River. This basalt, which is of a pentagonal form, occurs 

 frequently farther west, walling high plains or declivities, but 

 has then a porous crust-like surface. 



2nd group, or Detached Granite Mountains on the uppermost 

 forks, or different arms of the Platte River; they seem to be the 

 summits of mountain chains ; are remarkably smooth and 

 rounded, less so, however, towards the south-west. Their in- 

 clination is various, increasing upwards with every successive 

 layer westward, until they become vertical and inclining 

 again.* The granite itself is in every other respect like that 

 in the mountains ; A. — The first locality on this route i s 

 at the Sweet Water River, and commences with Rock Inde- 

 pendence. 



Srd group, comprising the Black Hills, or Black Mountains, 

 with the hills of the north and south forks of Platte River. 

 Consisting of: 



A. — Mountainous masses of conglomerate, without order, 

 but always deeply reclining on their still higher syenitic or 

 pseudo-basaltic ranges, which have broken through the body 

 of this vast group, and are thrown in confusion. In its 



• The low smooth granite masses which lie exposed along the upper, 

 have the same inclination, which are likewise syenite, but unbroken 

 for a short space, afterwards overcast again by pieces 



