56 TIk- Amei'icori (iroliXJIxt. .lanniiiy, 1893 
inenttirj- l)anding. Tlie acid rocks are lound to belong to tlie group 
of rhyolitix. The basic rocks, which prove to be hamlt^i, occupy an 
area only about half as large as the acid ones. The former have been 
more generally sheared into slates and more altered than the latter, 
but sufficient of the original structure is left to show their volcanic 
origin, and in cliemical composition they agree with normal basalts. 
Another i)roof of their igneous origin is that in places these rocks oc- 
cur as dykes. 
The age of the Soutli mountain volcanics and tlieir relation to the 
sandstone, in whicli AValcott has recently identified a lower Cambrian 
fauna, are briefly discussed and it is stated that the entire absence of 
sandstone inclusions in the lava and breccia, the observations of Keith, 
Geiger and Walcott, and the sections made by iliss iJascom across 
Monterey peak, Pine mountain. Jack mountain and Haycock all indi- 
cate that the sandstone is altogether above tiie volcanics. "The 
8outh mountain volcanic rocks therefore l)ecome, not merely in their 
petrographical character and richness in metallic copper, but also in 
their stratigraphical position, comparable with the Keewenawan or 
Nipigon series of lake Superior." 
Extensive chemical changes, involving devitrilicaticjn and the forma- 
tion of new minerals, have gone on in the volcanics of South mountain, 
but their original structures are finely preserved. Tlie formation of 
epidote has taken place to a great extent in the basalts. 
It seems possible, and indeed very probable, that many more areas 
of old volcanic rocks will be recognized in other regions of America* 
when they come to be carefully studied. The rocks of South moun- 
tain have been known for many years and yet their true character 
was not discovered, and it is not going too far to suppose that many 
other rocks of similar characters have been overlooked and will in 
the future be given their proper position. In this paper Prof. Wil- 
liams has not only given us some very interesting facts, but has also 
thrown considerable more light on the history of our earlierf ornuitions. 
,1 (■(iiifrihittitin to the (rroloi/i/ of tin' (J^rral I'Iniiis. l>y K(U!KKT H*.\Y 
Bulletin, G. S. A., vol. in, pp. 519-521, with a general section on the 
102d meridian. Two noteworthy features in the topography of the 
mid-Plains region are here noted. A valley lies between the margin 
of the plains and the Rocky mountains, the former having a steep 
western escar]jment from near Pueblo to near Cheyenne, while west- 
ward from Cheyenne a ridge, constituting the highest part of the 
plains, runs up to nearly 7,000 feet and abuts on the tilted Mesozoic 
and Palitozoic formations of the mountains. The other feature speci- 
ally described consists in the deep and very irregular erosion of the 
valleys in Nebraska and Kansas, between the Platte and the Arkan- 
sas, where sandy Tertiary beds are "carved into fantastic forms of 
castles and buttes and palisades whicli vary l)y a local picturesqueness 
the intense monotony of the plains." 
