30 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



Large masses of iron pyrites, some with brilliant crystalline forms, 

 were found ; others mixed with bits of charcoal and large masses of 

 petrified wood, showing the vegetable structure with great distinctness. 



Bones of some extinct saurian animal are frequently found in these 

 beds. In the sandstones of the upper bed many impressions of leaves 

 similar to those of our existing forest-trees are found. They comprise 

 the cinnamon, fig, laurel, sycamore, sassafras, magnolia, and many others 

 belonging to a genera common to both tropical and temperate climates, 

 but all belonging to extinct species. 



Indeed, the Cretaceous i^eriod marks the dawn of the existence of 

 dicotyledonous trees, or those similar to the existing forest, fruit, or 

 ornamental trees on our planet, and consequently forms a new and most 

 important era in the progress of American geological history. 



I shall have more to say in regard to them in my description of the 

 geology of other counties. 



These sandstones continue up the Little Blue until we arrive within 

 four miles of the mouth of the Big Sandy, when masses of a whitish lime- 

 stone make their appearance on the summits of the hills, and eight or 

 ten miles west of the Big iSandy these rocks assume an important thick- 

 ness. 



They are composed of bivalve shells, {Inoceramiis problematieus,) 

 which are as closely packed together in these rocks as if they had been 

 submitted to pressure, with enough carbonate of lime to cement the 

 shells together. The settlers find it useful for building-stones, but more 

 useful for converting into lime. It is a chalky shell-limestone and burns 

 into the best lime of any rock in the State. Whether it will be found 

 in great quantities either in the valley of the Little or Big Blue Eiver 

 remains still to be determined. 



On account of the hostility of the Indians in that region, I did not 

 think it safe or prudent to extend my examination more than about 

 eight miles above the mouth of the Big Sandy. 



The same rock occurs on Swan Creek, Turkey Creek, and the Big Blue 

 above the mouth of Turkey Creek. This rock was first studied on the 

 Missouri River, and first appears capping the hills about 30 miles below 

 Sioux City, Iowa, and extends to the foot of the Great Bend, near 

 Yankton, the capital of Dakota Territory. It occupies the whole country 

 to the exclusion of all other rocks, and a portion of it assumes the ap- 

 pearance of chalk. It has been hitherto supposed that the chalk of 

 commerce is not found in any portion of America, and although this 

 rock has the appearance and nearly the chemical composition of impure 

 chalk, the formation itself has not yet been clearly shown to be the 

 geological equivalent of the true chalk-beds of Europe. 



On the Missouri Eiver this formation covers an area about 200 miles 

 wide and 400 long. The Cretaceous rocks in the valley of the Missouri 

 were, several years ago, separated into five divisions by Mr. Meek and 

 the writer, and were for a long time designated by numbers, as 1, 2, 3, 

 4, and 5. 



In a paper published in the proceedings of the Academy of Natui-al 

 Sciences of Philadelphia, December, 1860, we published a general sec- 

 tion of the Cretaceous rocks of the Northwest. 



The sandstones which we have referred to in this report we designated 

 the Dakota group, or Formation No. 1, because these rocks were then 

 supposed to reach their largest development along the Missouri River 

 near Dakota Territory. 



Formation No. 2 was called the Fort Benton group, having its 



