112 PROCEEDINGS OF THE ACADEMY OP 



spicuous on the summits of the Bluffs ten miles above Iowa creek. At Dori- 

 on's Hills it reaches to the water's edge and is the prevailing formation from 

 thence to the foot of the Great Bend, where it passes by a gentle dip beneath 

 the water level of the Missouri. At Dorion's Hills there is a fine section of 

 this bed about eighty feet exposed above the water's edge, containing its 

 most abitndant and characteristic fossil, Ostrea congesta. In many places, as 

 opposite the mouth of Running Water, it assumes the form of a long series of 

 precipitous bluffs, giving a pleasing variety to the general monotony of the 

 scenery. This is one of the principal characteristic external features of this 

 formation. 



The upper portions of this rock is a yellowish and gray calcareous marl, very 

 soft and yielding, so that it is easily cut up into numerous ravines by the 

 temporary streams, and thus the bluffs along the Missouri often present the 

 appearance of a seriesof cones split from apex to base. The lower stratum, 

 however, is more com2)act and forms a soft bluish gray limestone. 



Though so well developed and covering so wide an area, the middle and 

 upper portions, at least, of this rock can never be made useful for building pur- 

 poses. Quite soft and friable in places, when detached, it absorbs moisture 

 rapidly and crumbles in pieces. Being a rich calcareous marl, it may be used 

 at some future time as a fertilizer. 



The fossils of this formation, though belonging to few species, so far as is 

 yet known, are numerous in individuals. A species of oyster (0. congesta) 

 is found in great quantities throughout the bed, and in localities Inoceramiis 

 problematicus is abundant. Fish remains, though consisting mostly of scales 

 and obscure fragments, are disseminated throughout the deposit, several species 

 of which have already been identified and described by Dr. Leidy. Entire 

 thickness of this bed about one hundred and fifty feet. 



Xear First Cedar island, a very singular bed makes its appearance super- 

 imposed on No. 3, which we shall consider as probably forming a local upper 

 member of that formation. It extends up the Missouri river to a point near 

 the Great Bend, a distance of about eighty miles. Lithologically it is a dull 

 black, unctuous clay, destitute of any grit, and does not effervesce with an 

 acid. It contains some carbonaceous matter and great quantities of Selenite 

 in crystals. 



Formation No. 4 of Vertical Section. 



This Formation is the most important one in the Cretaceous System 

 of the North West, not only in regard to its thickness and its geographi- 

 cal distribution, but also in its influence on the topographical features 

 of the country. It is only second in interest to the succeeding bed in the 

 number, beauty and variety of its organic remains. Commencing about ten 

 miles above the mouth of James river, where it is seen only in their outliers 

 capping the distant hills or bluff's, it continues gradually assuming a greater 

 thickness as we ascend the Missouri until reaching the Great Bend, where it mo- 

 nopolizes the whole region, giving to the country underlaid by it a most gloomy 

 and sterile aspect. At the Great Bend it attains a thickness of two hundred 

 feet, and continues to occupy the country bordering on the Missouri to the 

 mouth of Grand river, where, in consequence of the dip of the strata, it passes 

 gradually beneath the water level of the river. 



After dipping beneath the water level between Grand and Cannon Ball 

 rivers, this formation again rises to the surface about thirty miles below the 

 mouth of Milk river, (far up towards the sources of the Missouri,) by a reversed 

 dip of the strata from beneath the northern portion of the Great Lignite Basin, 

 as will be seen by reference to the section on the map. Near the mouth of 

 the Muscle Shell river it occupies the whole country for a distance of about 

 eighty miles, and thins out upon the tops of the hills near the mouth of the 

 Judith river. 



On the Yellow Stone I observed Formation No. 4 in but one locality, about 

 eighty miles above the mouth of that river. It does not attain a great thick- 



[May, 



