38 The American Geologist. January. i904. 
bro exposure than about half a mile; therefore the relation 
between the two rocks has not been demonstrated. Laborious 
and expensive borings would be necessary to do so. There can 
be little doubt, however, that the gabbro is an eruptive later 
than the quartzyte like the olivine diabase near Corson and a 
rock struck near Dover,* 500 feet down, which is much like 
the latter, a green eruptive struck at Yanktoni at 880 feet 
aind penetrated 49 feet, and a quartz-porphyry found at Hull, 
Iowa. % 
The rock is remarkably uniform in character, there being 
no blotching nor difference in size or arrangement of grains 
on different sides of a block or in blocks from different points. 
The grains are rarely over an eighth of an inch across. The 
combination of the black augite and magnetite with the pearl 
gray plagioclase is suggestive of delicate foliage showing clus- 
ters and branches. Quartz is absent and biotite is not evident. 
It takes a beautiful polish. The whole surface is lustrous, 
though the magnetite shows a duller metallic sheen, which 
gives another pleasing contrast. The writer failed to find 
trace of titanium, as Merrill reported, but can corroborate his 
other statements. 
From the abundance of iron present it has been feared that 
the rock would rust and disintegrate readily on exposure to 
weather, but the appearance of the natural exposures seems to 
demonstrate the very opposite. No glaciated surfaces were 
found ; the rock was probably too low below surrounding ex- 
posures to be strongly rasped by the ice, if touched at all. But 
the rock where exposed naturally near a small stream has a 
grayish appearance with some rust, and the weathered cor- 
tex is very thm, less than a sixteenth of an inch, and of very 
even thickness, its inner surface merging abruptly into rock 
as black and firm as that many inches below. All grains seem 
to have similar resisting power ; none seem to invite disinte- 
gration h\ absorbing corrosive elements. Where the rock has 
been covered with a few feet of porous silts and gravels, so 
that air and moisture have had most favoralile opportunity for 
corroding, the weathered shell is rarely over half an inch thick 
and has the uniform thickness and abrupt change below as in 
* Bulletin 2, S. D Geol. Survev, p. 98. 
t Ibid., p. 101. 1898. 
t Iowa Geol. Survey, vol. i, p. 165. 1892. 
