208 GEOLOGY OF THE BLACK HILLS. 



not as a sloping table but as a low anticlinal which gradually descends 

 and fades away to the southward. Tlie form of the prolongation is realized 

 by the outcrop of the Dakota sandstone which ceases at that point to be a 

 sim])le nionoclinal ridge and spreads out into a triangular table sloping to 

 the southeast and southwest. Here again a glance at the bird's-eye view 

 will give a more definite idea than can be conveyed by a description. 



At the north, or rather at the northwest, for that is the direction of the 

 other extremity of the axis, there is a similar prolongation of the u})lift, but 

 it is on a far greater scale. An anticlinal nearly as broad as the main 

 uplift extends for at least forty miles beyond the end of the table, and is 

 itself somewhat tabular. Where it adjoins the main table it is decidedly 

 lower, and the two surfaces are connected by a gentle flexure. Thence it 

 descends slowly to the northwest, and it probably merges gradually with the 

 datum plane in that direction Our observations, however, did not extend 

 to its limit. On its northeastern edge it is limited by a definite flexure, 

 which in its southward prolongation merges with tlie eastern flexure of the 

 main uplift. Its southwestern limit shows no such flexure and is not well 

 defined. 



The extreme height of the uplift as referred to its datum plane — that 

 is the maximum amount by whicli a stratum in the arch of the Hills rises 

 above its prolongation in the surrounding Plains — is estimated to be G,GOO 

 feet. This height is found in the crest of the main uplift, near tlie eastern 

 edge of the tabular top. The western edge of the tabular top is about ><00 

 feet loww. The altitude of the southward anticlinal extension, where it is 

 crossed by the Cheyenne River, is about 1,800 feet above the datum. The 

 northern prolongation, where it joins the main uplift, has a central height of 

 about o,.'iOO feet, just one-half of the full height of the main uplift. Where 

 it is crossed by the Belle Fourche River its height is 2,200 feet. The total 

 volume of matter here lifted above the general surface, or, in other words, 

 the solid contents of the restored dome of rock, is 4,200 cubic miles. 



In this description no allusion has been made to the displacements 

 associated with the igneous rocks. Those displacements modify the form 

 and destroy the simplicity of the great displacement, but they have no 

 discernible relation to it except the relation of superposition. Whether 



