ELKHEAD MOUNTAINS. 177 



friable sandstones were seen in contact with a basaltic breccia, or ash, 

 which rested on it with an unconformity of erosion, rather than of angle 

 of deposition. Although these sandstones, where seen, show little to dis- 

 tinguish them, the occurrence of fragments of coal in the wash of the 

 stream renders it probable that they belong to the same horizon as the beds 

 observed along the banks of the Little Snake River. 



Near the head of Slater's Fork, a dike of columnar basalt connects the 

 main ridge with the sharp narrow peaks between this valley and that of 

 the Little Snake Eiver. Their flows cover the greater part of the surface 

 of the country, and only in the deeper-cut valleys are the remains of the 

 underlying sandstones exposed. These basalts are compact, rather crystal- 

 line rocks, of dark-gray color and even texture, in which can be distinguished, 

 by the unaided eye, a great deal of olivine and augite; only by the aid of 

 the microscope can the magnetite, nepheline, or plagioclase-feldspar be 

 detected. Of these peaks, that east of Slater's Fork shows also a remarkably 

 fine columnar structure, while, in a low flat-topped hill, opposite the mouth 

 of Battle Creek, something of the shape of an ancient crater is suggested. 

 Out of the flat mesas to the north of the Little Snake River rise two 

 basaltic peaks. Of these. Watch Hill is a wedge-shaped mass, of no great 

 height, standing close to the river, and in near connection with the flows 

 on the south of it. Its rock is a dark-gray dolerite, containing plagioclase- 

 feldspar, augite, olivine, and magnetite, as seen under the microscope, with 

 a great deal of dark globulitic substance between the crystals. 



Bastion Mountain is a flat-topped peak, whose horizontal outline, owing 

 to a deeply-cut valley on the northeast side, has the shape of a U. Its 

 summit is for the most part covered with forest, and its sides in the upper 

 portion present precipitous walls, which in many places are inaccessible. 

 The basalt of which it is composed is a light-gray porous rock, the cavities 

 having, in the hand-specimen, a parallel arrangement, which give the efiect 

 of a wavy, rudely schistose structure to the rock. These cavities, Avhich 

 are very small, contain a fine yellowish incrustation of carbonate of lime. 

 The augites in this rock are large and distinct, and olivine can be ea,sily 

 distinguished by the unaided eye. The microscope discloses as well biotite, 

 magnetite, nepheline, and a comparatively large amount of plagioclase, and 



12 D G 



