THE GEOLOGY OF NORTH DAKOTA 9 



the bottom along with the clay to form the calcareous shale or 

 chalky marl of this Cretaceous deposit. 



The percentage of lime carbonate in the different layers varies 

 widely, ranging from 30 per cent and less to 75 per cent. Many of 

 the beds are suitable for making natural hydraulic cement and are 

 used for this purpose, while certain layers have almost or quite 

 the composition of a natural Portland cement rock. 



Wherever the Niobrara formation is exposed in the Pembina 

 Mountains it maintains a fairly uniform character throughout its 

 thickness of 150 feet and more. By far the greater portion of the 

 aggregate thickness is formed of a rather dark bluish-gray speckled 

 rock, which commonly varies in lime carbonate content from 55 

 to 65 per cent in passing from one layer to another. Generally 

 the more speckled the rock appears the higher it is in lime. Between 

 these thicker beds high in lime carbonate are others much thinner, 

 varying from a few inches to a foot in thickness, which are much 

 lower in lime. 



Where exposed in northeastern North Dakota the Niobrara 

 strata have yielded a number of vertebrate and invertebrate fossils. 

 Among the latter are Inoceramus labiatus, specimens of Ostrea and 

 Avicula, besides the microscopic forms previously mentioned. The 

 large diving bird, Hesperornis, several species of fishes, Plesiosaurus, 

 and the vertebrae of a crocodile have also been found. 



The maximum thickness of the Niobrara in the Pembina Moun- 

 tains is 165 feet. At Morden, Manitoba, the formation is 160 feet, 

 and farther north it averages from 150 to 200 feet, but seems to 

 thicken toward the west, for the well at Deloraine, a few miles 

 north of the Turtle Mountains, shows a thickness for the Niobrara 

 of 545 feet. The formation probably has a thickness of at least 

 400 feet in southern and central North Dakota, since at Valley 

 City the combined thickness of the Benton and Niobrara is about 

 900 feet. The artesian wells at this place reach the Dakota sand- 

 stone at a depth of approximately 800 feet below the bottom of the 

 Cheyenne Valley, and the calcareous shale, which outcrops in the 

 sides of the valley and is believed to be Niobrara, rises about 80 

 feet above the well curbs. The thickness of the beds between the 

 top of the Dakota sandstone and the base of the Pierre is thus 



