ISTKLXTriMO AND ACH. 203 



proximity to the igneous matter. Eacli peak has tlio same general conical 

 form so often described, and a simihir system of cleavage planes was also 

 observed. Some cross-planes give locally an api)earance of columnar 

 structure. The rock is a greenish gray trachyte similar to that of the Bear 

 Lodge, but though it contains many crystals of feldspar it is not so highly 

 crystallized as the I'ock from the latter peak. At the base of the buttes in 

 one or two localities a rock was found exceedingly^ light and cellular in 

 structure, yellowish in color, and very like a volcanic tufa. As the result 

 of a microscopic examination Mr. Caswell designates it a rhyolitic breccia, 

 including fragments of both sandstone and rhyolite. 



SECTION XI. 



STKUCTURE AND AGE OF THE BLACK HILLS. 



It remains to consider the origin of the topographic forms which con- 

 stitute the Hills. How and when did the plateaus, and peaks, and cliffs, 

 and caiions, and valleys that make up its topography come into existence ? 

 The answer to the question is, in general terms, that the rocks were uplifted, 

 and that being uplifted they were by the processes of erosion worn away 

 and carved into the forms we have seen. If the form of the uplift had 

 been different, if the order and character of the rocks had been different, 

 if the date of the uplift had been different, there would have been a differ- 

 ent topographic result. A complete analysis of the resultant forms, taken 

 in connection with the structure of the rocks and viewed in the light of tlie 

 natural laws of erosion, should tell, first, the form of the displacement or 

 uplift and, second, the date of its occurrence. Our analysis is unfortunately 

 not so full as to affoi-d either of these results in a satisfactory way, but it 

 will at least give a fair idea of the methods by which they may be reached. 

 In presenting the material and deductions the following order will be 

 observed : 



First, the form of the displacement wnll be considered; second, the 

 relation of the topography to the form of the displacement and to the rock 

 structure will be discussed ; and third, the evidence which bears upon the 



