LOGAN. | The Upper Cretaceous. 233 
Prionocyclus wyomingensis somewhat abundant. From 20 to 50 feet 
from top of formation, many calcareous nodules ranging from a 
few inches to 5 feet in diameter, having a cone in cone structure 
and well formed calcite crystals on the inside. Thickness, 175 to 
200 feet. Total thickness of Fort Benton, 400 to 450 feet. 
Niobrara.-—l. A series of limestones and calcareous shales with 
prevailing blue colors. Limestone series at base, 50 feet. Individual 
beds range from a few inches to 3 feet. Separated by layers of 
gray shales 1 or 2 inches thick. Limestone light gray color, creamy 
white on weathered surfaces, compact, rather fine grained. Lower 
layers contain nodules of iron sulphide. Characteristic fossils, 
Inoceramus deformis covered with Ostrea congesta. Upper limit of 
limestone passes into light gray limy shale, terminating in two layers 
of chalky limestone. Thicness, 175 feet. 
II. Chiefiy an argillaceous, laminated shale, dark gray colors, 
acquiring a yellow color and rougher texture on weathering. Thin 
plates of gypsum somewhat abundant, and oval fish scales and 
fish bones. Calcareous concretions at various horizons, broadly 
ellipsoidal, contain crystals of barite. Total thickness of formation, 
500 feet. 
Fort Pierre. 
-Deposits of laminated argillaceous shales not in- 
terrupted by sandstone; limestone or other hard layers. A lower 
zone of medium gray color, texture rough. Barren of fossils. Con- 
tains few concretions. Thickness, 400 or 500 feet. 
A zone above characterized by abundance of ovoid concretions, 
carbonate of lime. Contain fossils. Thickness, 600 feet. 
A zone above, shale, fine textured, medium gray color, with con- 
cretions larger than in zone below, contain fossils. Thickness, 1000 
feet. 
CENTRAL COLORADO SHCTION. 
Baim SivancOnsr 
Dakota.—A thin bed of conglomerate, composed of well rounded 
pebbles, united by a strong silicious cement; hard, usually gray 
sandstone, in two prominent benches separated by bands of fire 
clay; fossil plants abundant in some localities. 300 feet. 
Benton.—Dark shales with frequent indications of fossilliferous 
*T. W. Stanton: Colorado Formation and its Invertebrate Fauna, A. J. 8. (4) 
vol. I, p. —, 1896. 
