IGNEOUS INTRUSIONS IN THE BEACK HILLS 29 
Little Sun Dance dome.—This hill, sometimes called Green 
Mountain, is situated about two miles northeast of the town of 
Sun Dance, the county seat of Custer county, Wyoming, and 
about three miles north of a larger uplift of the same character, 
known as Sun Dance hill. 
A view of Little Sun Dance as seen from the topographic 
monument on the summit of its more imposing companion, is 
given in Fig. A, Plate I. This dome is remarkably regular from 
whatever direction it is seen, and is composed of stratified rocks 
which have without question been elevated by a force acting 
from below directly upward. The summit of the hill consists of 
Carboniferous limestone, the crown being an unbroken dome. 
About this inner dome and dipping sharply in all directions are 
strata of purplish limestone belonging to the Red Beds or Tri- 
assic system. This resistant layer has a thickness of about 
twenty feet,and was once continuous over the top of the dome, 
but has been removed from the summit by erosion, and several 
radiating gorges cut by streams in the portion of the envelope 
which remains.) Between the drainage lines the strata extend 
far up on the sides of the dome so that the edge of the outcrop 
when followed about the hill forms a zigzag line. As described 
by Newton, the exposed edge of this limestone resembles the 
broken edge of a piece of paper that has been punctured by a 
sharp pencil. Although the strata have been bulged upward, 
they have not been shattered, the apparent radial breaks being 
due entirely to erosion. Resting on the limestone, are soft, easily 
eroded shales, a hundred feet or more in thickness. The position 
of this bed is indicated about the base of the hill by a smooth, 
grassy valley a few hundred feet in breadth. On the outer side 
of this valley is a rampart, more or less well defined, of sand- 
stone layers, which dip away from the dome in all directions but 
present a steep inner escarpment. The sandstone and shale 
belong, together with the purple limestone, to the Red Bed sys- 
tem, and are the highest or youngest strata exposed in the 
immediate vicinity. 
The streams which have eroded deep trenches in the sides of 
