182 GEOLOGY. 



Mexico, which' spread over the area where the sediments of the 

 Dakota Group are now found. There can be no question about 

 the Dakota Group being a shallow sea and beach deposit. Just 

 such materials are now being deposited in existing shallow seas. 

 Examples can be seen along the North Sea, on the Belgian coast, 

 and along the shores of Holland where there are extensive muddy 

 flats composed of substances which if compacted would be similar 

 in constitution to the Dakota sandstone. Small grains of sand are 

 rolled up by the sea which are mingled with the mud deposits 

 brought down by the rivers. The rivers bring down iron held in 

 solution which is deposited in the presence of organic matter on the 

 bottoms, often giving the grains of sand a coating, which subse- 

 quently became loosely compacted sand rock with a rusty, red, or 

 brown color. " Marine animals, especially shells, are rare in deposits 

 of this kind." "It is shunned by every kind of land animals, and it 

 has therefore no other remains imbedded into its compound but 

 saurians and rarely fishes. It has no remains of marine plants be- 

 cause these do not grow on the soft ground." (Lesquereux.) 



Prof. Marcou and Capellini regarded the Dakota Group as a 

 fresh water deposit. If the considerations already adduced are cor- 

 rect it cannot possibly have such an origin. It can also be added 

 that at Sioux City, and in the bluffs in Dakota County are found 

 mingled with dicotyledonous leaves peculiar -to this deposit such 

 marine shells as Pharella Dakotensis, Axinea Siouxensis, and Cy- 

 prina arenacea. Prof. Meek has also identified not less than twelve 

 additional marine species from this group in Kansas where the leaf 

 impressions are characteristically abundant. Its very extent con- 

 tradicts such an opinion. It is from sixty to one hundred miles 

 broad and adjoining and overlapping the Carboniferous and Per- 

 mian, it extends from Texas through Minnesota to, and probably 

 through British America to Greenland. It is not conceivable that 

 there should be a fresh water deposit of such extent. The homo- 

 genous character of its materials also contradicts this view. No 

 American geologist, however, ever entertained this opinion. 



Discussions tn Regard to the Character and Age of the Dakota 

 Group. No group of remains have ever excited more, and few as 

 much interest as that of the Dakota. When first studied in Kan- 

 sas, it was regarded by Prof. Hawn and Swallow as of Triassic 

 age. Afterwards it was pronounced Jurassic by Prof. Marcou. 

 "In the mean time Dr. Hayden sent some sketches of the leaf im- 



