Page:The Journal of geology (IA journalofgeology21894univers).pdf/264

As is well known, the Black Hills District was surveyed by the party in charge of Professor W. P. Jenney in 1875, the geological report being written by Professor Henry Newton, whose death occurred two years later. The report, edited by Mr. G. K. Gilbert, was published in 1880 by the U. S. Geographical and Geological Survey of the Rocky Mountain Region in charge of Major J. W. Powell. The peculiar and interesting geological features of this remarkable outlier of the Rocky Mountain range need to be set forth here further than to say, that on all sides after leaving the centreal nucleus of eruptive rocks sedimentary deposits occur with diminishing dip in an ascending geological order. First, there is a narrow ring of Potsdam sandstone; then a wide belt of Carboniferous limestone; next an encircling trough, aptly compared by Professor Newton to a moat, of red sandy gypsiferous clays, in which is included a purple limestone terrace, all of which is supposed to be Triassic and to be the equivalent of the "Red Beds" of more southern regions; skirting this is a very narrow border of highly fossiliferous light colored Jurassic clays or marls; then come the foot-hills, which consist of Cretaceous sandstones and shales referred by Professor Newton to the Dakota, No. 1, of Meek and Hayden's section; these slope back to the dark shales of the Fort Benton group, which are succeeded by higher Cretaceous beds that extend to the plains and pass under the Bad Lands of the White River formation.

The belt of Cretaceous, which lies outside the Red Beds and Jurassic and forms the foot-hills, constitutes an elevated rim with an escarpment at its inner margin rising abruptly above the Triassic trough, the Jurassic exposures being often confined to the lower part of the escarpment. This cannot be better rep-