GEOLOGY 219 



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

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

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



" The specimens of selenitc obtained from this division of 

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

 peculiar forms that they assume — some of them presenting 

 the 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 sclenite. Others are 

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

 lance," lanciform, radiating, &;c. 



" 4lli. 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 with 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. 

 ]5ut these lumps are more frequent in the lower half of the 

 depositc than in the upper, and fuially cease altogether to 

 appear. 



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



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