Dikes of the White Rirer Mioceite. — Case. 249 
with water, this water maj^ be under hydrostatic pressure and 
certainly is under that of the overlying strata, so if diWj crack 
is opened from above to a saturated layer of loose sand the 
water will rush up and carry with it the sand. The subse- 
quent hardening of this sand will produce the sandstone dikes. 
In evidence of the volcanic origin of the cracks occupied by 
the sandstone, the author of the paper calls attention to the 
parallelism of the dikes as illustrated in fig. 2, p. 413 of his 
article (op. cit.). 
Later, an article appeared by Prof. Kobt. Hay on the "Sand- 
stone Dikes of Northwestern Nebraska" (Bull. (4eol. Soc. Am., 
vol. Ill, p. 50). The dikes here described are similar to those 
described by Uiller but are fewer in number, only two being 
noted. They are in the vicinity of Chadron, Nebraska, and 
pierce the clays of the so-called Pine Kidge. Prof. Hay re- 
gards them as intrusive from below, as will be seen from the 
folk>wing quotation from his article: 'Tt (the main dike) 
does not on either side reach the top of the ravine, and a bluff 
of much greater elevation a few hundred feet away shows no 
sign of its presence, so it ma}' be definitely regarded as hav- 
ing been formed before the completion of the soft clays and 
m^rls. * * One of the evidences of intrusive character lies 
in the structure of the laminated sheets on either side of the 
dike. In these the laminte farthest from the dike are more 
argillaceous than inside and the inside lamiiue are decidedly 
grooved, with vertical ridges, and grooves to correspond on 
the sides of the wall itself." 
Prof. Hay does not attempt to explain the origin of these 
dikes but suggests that "these dikes may be related to the 
phenomena of mud volcanoes, as they were certainly intruded 
from below, and they may be expressive of the closing period 
of the Black Hills uplift." He also speaks of the existence of 
other dikes in the "bad lands" of South Dakota. It is the aim 
of this article to describe these dikes and show how the^^ dif- 
fer from those already described and possibly throw some 
light upon the formation of sandstone dikes in general. 
The "bad land" or White River Miocene region of South 
Dakota is tlie seat of the eroded remnants of a vast lacustrine 
deposit of lower Miocene or Oligocene age. The entire thick- 
ness, according to Hatcher (Am. Nat., March 1898. p. 21S ). is 
