AKTESIAN WELLS IN NORTH AND SOUTU DAKOTA. 533 



descends at least, 40 feet, but the descent is more if the well at Fort Ran- 

 dall is at a considerable height above the Missouri River. In 25 miles from 

 Tyndall eastward to Yankton, the water head sinks probably 150 or 200 

 feet. In the next 22 miles eastward to VermiUion the descent is 140 feet. 

 This feature of the artesian water supply is caused, as before stated, by its 

 outlets through springs in outcrops of the Dakota sandstone, which begin 

 30 to 40 miles southeast of Vermillion and extend tlience southeast and 

 south. 



All the eastern outcrops of the Dakota sandstone are lower than the 

 upper portions of the James River basin and the wells farther west at High- 

 more and Harold. These outcrops, therefore, can not be the sources from 

 which the sandstone receives its artesian water, but, as we have seen, they 

 are the avenues of its natural outflow. We must look instead to the western 

 outcrops of this formation, where it skirts the Black Hills and exposes its 

 upturned edges along the base of the Rocky Mountain ranges, for the areas 

 upon which the water is carried downward into the sandstone. Thence we 

 know tliis stratum to be continuous beneath the plains to the James River 

 Valley, for there are no nearer or other inlets from which the copious sup- 

 ply of the artesian wells can come. At a plane of similar or greater depth 

 an artesian reservoir exists beneath much, if not all, of tlie country west- 

 ward to the mountains. The gradients of the altitudes to which the water 

 of wells is capable of rising along east-to-west lines in South Dakota, as at 

 Huron, Miller, and Highmore, are approximately the same as the average 

 westward ascent of the country, demonstrating this western origin of the 

 water supply, and indicating tliat such wells may be obtaineil u])on an 

 extensive region of the arid plains. 



The quantities of alkaline matter and salt dissolved in tlu^ water of these 

 wells usually give it a brackish taste, and make it unlit for drinking by 

 people and for ordinary domestic uses; but it is drunk freely by cattle and 

 horses, with no unfavorable effects. These mineral ingredients seem to have 

 been derived from the Cretaceous shales, and probably in part from lieds 

 in the Dakota formation, with which the water has been in contact during 

 its slow percolation Imnilreds of miles through the sandstone. They are 

 the same in kind and similar in amount with the mineral matter of Devils 



