90 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



Dakota Group disappears beneath the water-level, and is succeeded by 

 a series of black, plastic, laminated clays, with lighter-colored arenaceous 

 partings and thin layers of sandstone. Near the mouth of the Ver- 

 million Eiver the upper portion becomes more calcareous, and gradually 

 passes up into the next group. This formation has been called No. 2, 

 or Fort Benton Group. It is often immensely thickened in the vicin- 

 ity of the mountains from the north line to New Mexico, but on the 

 Lower Missouri, where it was first observed by geologists, it never 

 reaches a thickness of more than one hundred and fifty or two hun- 

 dred feet. In New Mexico it occurs as the most conspicuous of the 

 cretaceous divisions, and along the line of the Kansas Pacific Railway, 

 in Kansas, it has yielded large quantities of the most remarkable rep- 

 tilian remains. In the chapter on the geology of that route I shall 

 have occasion to dwell more minutely on the interesting facts connected 

 with this group. On the Missouri Eiver it has yielded a number of 

 species of Inoceramus, Scaphites, Amonites, and some thin layers are 

 made up of remains of the scales and teeth of fishes. Farther up the 

 Missouri River, near the mouth of the Niobrara, and resting on these 

 sandstones and clays, is a thick bed of chalky limestone, containing 

 vast quantities of a small species of oyster, and a large bivalve, Ino- 

 ceramus problematicus, which is identical in species, or closely allied 

 with one found in many portions of Europe. Some remarkable forms of 

 fishes, not unlike our shad or herring, also sharks' teeth, have been 

 found in abundance. A few other shells have been described in various 

 localities in this chalk, and all of them are of a strictly marine charac- 

 ter Much of this limestone, though colored extensively with oxide of 

 iron, is soft, and leaves a mark on a blackboard or cloth like our com- 

 mon chalk of commerce. It is also composed largely of infusorial 

 remains, as distinctly shown under the microscope. This formation, as 

 well as the sandstone, is very widely distributed over the plain country 

 in Nebraska, Dakota, and Kansas, and its influence on the agricidtural 

 prosperity of these regions is very great. The fertility of the soil is 

 largely due to the calcareous matter of the one mingled with the silica 

 derived from the other. The bluff-like character of these chalky lime- 

 stones, as shown along the channel of the Missouri from the mouth of the 

 Niobrara to the mouth of White Eiver, is well illustrated by Figure 2. 



Fig. 2. 



Bluffs of Niobrara Group, or Cretaceous No. 3. 



This is one of the most interesting of the cretaceous divisions. It is 

 found in some form wherever the cretaceous beds occur, from the north 



