GEOLOGY 219 



of a yellowish color at the bottom, becomes black and more 

 foliated in its superior beds. The selenite is more abun- 

 dant, replacing, as it were, the white indurated clay. 



" The specimens of selenite obtained from this division of 

 the Dixon group are Avorthy of notice, in consequence of the 

 peculiar forms that they assume — some of them presenting 

 tlie appearance of leaves of trees, beautifully and gracefully 

 scolloped ; which has encouraged me to venture upon a 

 descriptive name, as a mineralogical variety, by which to 

 designate them. I call them phylloidal selenite. Others are 

 in the usual shape of six-sided regular prisms, ' en fer de 

 lance," lanciform, radiating, &c. 



" 4th. The rock designated as D is the last member of the 

 trans-Missisipian cretaceous formation, as it presents itself on 

 the Missouri River. It is a vast deposit of plastic clay, about 

 two hundred feet thick, which may be considered, however, 

 divided into two equal parts by a stratum of argillaceous car- 

 bonate of lime in nodules, of which I had no occasion to 

 ascertain the thickness. Many of these nodules, having 

 fallen from their original position, are met with in consider- 

 able quantities in the beds of the ravines, and in other 

 places. Associated w^ith it is a ferruginous sandstone, 

 which presents itself in flat polygons, on the surface of which 

 there are seen numerous concentric lines of great regularity, 

 so as to imitate the transverse sections of a tree. The same 

 deposit contains, disseminated through it, lumps of the yel- 

 lowish clay of the inferior stratum, C, and enclosing leaves 

 of selenite, and cavities lined with concretionary gypsum. 

 But these lumps are more frequent in the lower half of the 

 deposite than in the upper, and finally cease altogether to 

 appear. 



" There are also found, throughout the clay deposit, loose 



