58 Feather stonkaugW s Geological Report. 



We have now reached the cretaceous group, where we 

 again find an appearance of equivalents in this country to the 

 European formations. The lower part of this group has been 

 usually called the green sand formation, from its abounding in 

 green-colored grains, which, upon analysis, give silex, pro 

 toxide of iron, alumine, and potash. They are the distinguish 

 ing mineral characteristics of its structure wherever it has been 

 observed, but do not obtain in the central portion of the form 

 ation, where a strong bed of arenaceous and calcareous clay, 

 called gaultj divides it into three portions, the lower green 

 sand, the gault, and the upper green sand. 



The lower green sand is very ferruginous, containing beds 

 of sand more indurated towards the bottom, with limestone in 

 some localities. 



The gault is a bed of grayish blue clay, effervescing 

 strongly, and containing fossils which have a very fine nacre 

 frequently. 



The upper green sand is a mass of stratified sands, contain 

 ing a large portion of the green grains, and occasionally green 

 or reddish nodules, which, upon analysis, have yielded phos 

 phates in great proportions. Thick regular seams of chert are 

 also sometimes found in it. 



The upper part of this group consists of the chalk marl, the 

 lower chalk, and the upper chalk, all of w r hich appear to be 

 deficient in the United States. The mineralogical characters 

 of these two last members of the cretaceous group are so well 

 known, the carbonate of lime of which they consist, on account 

 of its pure white color and its loose state of induration, being 

 so conspicuously different from any other known mineral, that 

 the student is in no need for any other guide to assist him in 

 discriminating between this and any other rocks he may meet. 

 They have not yet been noticed in any part of the United 

 States. This is not the case with the lower members of the 

 cretaceous gioup, which form a very extensive but very irreg 

 ular line from New Jersey to the neighborhood of the Mandan 



