30 The American Geologist. juiy, iggi 
outcrops l)e:iring a thick l)ed of massive gypsum. Loup Fork 
mortar-ledges, bearing teeth of Hippothcrium speciosum, etc., outr 
crop at mau}' phvces, and Quarternary deposits, yielding remains 
of Mastodon., occur along the larger streams and draws. Sand- 
hills, produced chiefl}' by the decomposition of the Loup Fork 
mortiir-rock and the drifting of its less soluble constituents, occup}- 
a lower portion of the slope on the north side of the Beaver. 
The leaf-bearing marl has a numl^er of outcrops south and 
southwest of Alpine. That especially- examined is a rather steeply 
inclined bed, closel}' related stratigraphicalh- to a ledge of ordi- 
nar}' Loup Fork mortar which yields teeth of Illppotherium. It 
is more or less stratified to laminated parallel with its inclined 
upper surface. Lithologically it is a white calcareous to siliceo- 
calcareous rock, composed of impalpably fine substance, and more 
or less indurated, according to the proportion of chemicall}' com- 
bined silica. The purest parts of this bed have the earthy frac- 
ture, soft smooth feel, easil}' finger-smoothed surface, and general 
physical appearance of chalk. Other parts have a ston}' hard- 
ness and the texture of fine-gi'ained limestone. Portions of neigh- 
boring outcrops are decidedl}' siliceous, resembling in texture lith- 
ographic limestone, or even chert. The harder varieties, in 
keeping with their highly siliceous character, break with conchoidal 
fracture. In the softer, and sometimes in rather hard varieties 
of the marl occur chert-like concretions, the larger of which reach 
a diameter of two or three feet. These are usuall}' sharp-outlined 
oblate spheroids of perfect symmetrj', though sometimes irregular. 
They break with conchoidal fracture into exceedingly sharp- 
edged segments, the fresh surfaces of which have the color of 
milk-trimmed coffee, bleaching to white under the weather. 
Attempts at usmg the rock for building purposes, b^' selecting 
intermediate varieties, have proved unsuccessful. The softer vari- 
eties crumble ; the harder are too brittle, cracking into conchoidal- 
faced segments upon prolonged exposure to the elements. There 
occurs, however, in this district, still another phase of rock of the 
Loup Fork which is more like ordinary stone in its relation to 
practical purposes. This is a compact siliceo-calcareous breccia, 
almost entirel}' composed of broken shells and siliceous shell-casts 
of a species of Phmorhis ; it answers fairl}' for well-curbing, foun- 
dations, etc. 
Certain ferruginous beds also were seen in this district, and are 
