150 Sioux City Academy of Science and Letters. 
member of the Benton group in southeastern Colorado. 
The dividing line between the Dakota and the Graneros 
is clearly marked by the plant-bearing sandstone bed. 
While lithologically the lower shale of the Graneros 
somewhat resembles the shales in the Dakota, paleon- 
tologically the two formations differ widely. The Benton 
possesses a fauna distinctly marine while the Dakota 
presents what have been considered both fresh-water and 
marine animal forms together with many plant remains. 
The Graneros shale is about 50 feet thick in the bluffs 
in Sec. 14, T. 29 N., R. 7 HE. It thickens to the north- 
westward, but it appears to thin toward the southeast 
and may entirely disappear in that part of the county 
southwest of Homer, where the fossiliferous Greenhorn 
limestone appears to lie very close to if not directly on 
the Dakota sandstone. The upper part of these shales 
yielded a few Inoceramns labiatus, and the lower shale has 
yielded reptilion remains, notably near Ponca, Nebraska, 
where more than 40 feet of the vertebrae of a mosasaur 
were found. The fossil lay below the Inoceramus beds 
and above the Ponca lignite seam, according to Mr. 
J. C. C. Hoskins, of Sioux City. 
The Graneros shale merges above into chalky beds 
which are overlain by slabby, sandy limestones, crowded 
so full of the fossil /noceramus labiatus that the rock splits 
easily along the planes formed by the shells. These lime- 
stone beds, now known as the Greenhorn limestone, form 
the ledge so prominent at Cedar Bluff on the Iowa side of 
Big Sioux River, and in Dakota County they extend near- 
ly the whole length of the escarpment, forming the tops 
of the hills between Hubbard and Homer, where part 
of their thickness has been removed by erosion. They 
partake of the general northwest dip and in Dixon 
County just above Ponca landing are only about 50 feet 
above Missouri River, passing below the water north- 
west of Newcastle. 
Relations of Greenhorn limestone formerly misunder- 
stood—The slabby, fossiliferous limestone beds in Da- 
kota County, Nebraska, and in Woodbury County, lowa, 
have long been mistaken for the upper formation of the 
Colorado group, the Niobrara, owing to their chalky con- 
tent and their resemblance in general to the latter. The 
fact that the true Niobrara beds first appeared in north- 
