IGNEOUS INTRUSIONS IN THE BLACK HILLS 25 
although some of the hills named, which had not been examined 
by the writer, may reveal this structure when more thoroughly 
examined. They differ, also, from volcanic necks like those of 
New Mexico described by Captain Dutton, to which some of 
them have a superficial resemblance, in the fact that the injected 
rock did not reach the surface so as to form either coulees or 
cinder cones. As they are composed of igneous matter forced 
into sedimentary strata and have a plug-like form, it will be con- 
venient to call them plutonic plugs. 
In the hills examined by the writer the structure has been 
revealed in-varying degrees by denudation, so that an examina- 
tion of a few examples furnishes a series of illustrations ranging 
from an unbroken dome of stratified rock arching over the sum- 
mit of a concealed mass of plutonic rock, to imposing towers of 
columnar rhyolite several hundred feet in height, exposed by 
the removal of the softer strata into which they were intruded. 
The first in this series is Little Sun Dance dome. This is a 
regular dome of stratified rock, about a mile in diameter, the 
outer layers of which have been removed and the inner ones of 
resistant limestone, deeply gashed by erosion, but not dissected 
sufficiently to expose the top of the igneous plug which presum- 
ably exists beneath. 
The other extreme is shown by Mato Teepee. In this instance 
the arch of stratified rock which once surmounted the summit of 
the plutonic plug has been completely removed and the sur- 
rounding strata eroded away. 
The rocks of which the plutonic plugs are composed have 
been studied microscopically by J. H. Caswell? and found to be 
rhyolite and sanadine-trachyte. 
The rock composing the several hills is closely similar in 
general appearance and in chemical composition and crystalline 
structure. It is described in the report just referred to, as light- 
*Mount Taylor and the Zuni Plateau. In 6th Annual Report of the U.S. Geo. 
logical Survey, 1884-5, Plates XI.—XXII. 
* Microscopical Petrography of the Black Hills of Dakota. In report of the 
geology and resources of the Black Hills of Dakota, by Henry Newton and Walter P. 
Jenney, Washington, pp, 489-527, Plates I.—-II. 
