REPOnT ON THE GEOLOGY OF NORTH AMERICA, 53 



glauconie sableuse of Brongniart. These sands occasionally 

 occur in indurated strata containing fossils, when they form a 

 rock ijrecisely the same in all respects as that which underlies 

 the limestone in Alabama. 2ndly. The peculiar greenish chlo- 

 ritic grains of the greensand formation of Europe. This mine- 

 ral exists generally in the shape of small grains of about the 

 size and form, and not unfrequently of the dark plumbago co- 

 lour, of gunpowder. Sometimes it has a rich warm green, but 

 more commonly an olive grey or dull blue, or even a very dark 

 chocolate colour. 



The grains, although they contain about 50 per cent, of silica, 

 are not gritty, can be easily bruised between the teeth, and when 

 moistened some varieties can even be kneaded into a somewhat 

 plastic mass. A pile of this marl, as the granular mineral is 

 called by the inhabitants of New Jersey, after being somewhat 

 exposed to the air, frequently contracts a light grey hue, from 

 the exterior grains becoming coated with a white inflorescence, 

 which, from some observations I have made, is most probably 

 carbonate of lime. The following analysis by Mr. Seybert pre- 

 sents a fair average of the composition of the green grains : — 

 silica 49-83, alumina G'OO, magnesia 1-83, potash 10-12, prot- 

 oxide of iron 21*53, water 9-80; loss 0*89 = 100 grains. Other 

 analyses show occasionally as much as 5 per cent, of lime. 



Mica in minute scales mingles not unfrequently in the less pure 

 varieties of the marl, which often contains more or less blue clay. 



Once or twice, in examining a mass of these mineral grains, 

 I have detected numerous minute spicula of selenite. Almost 

 every large heap of the marl exhales a distinct odour, closely 

 resembling sulphur. These mineral grains occur in greater or 

 less proportion in nearly all the strata, both arenaceous and 

 calcareous, of the formation ; but what is remarkable, they occur 

 alone, without any admixture of either sand or clay, in a homo- 

 geneous deposit, which seems to underlie nearly the whole secon- 

 dary tract of New Jersey, the stratum averaging ten or twelve 

 feet in thickness. 



It is this stratum which is especially called the marl, rather 

 from its highly fertilizing action upon the soil than for any re- 

 semblance it has to marl strictly defined. I am not aware that 

 the green chloritic substance has been found composing any 

 extensive separate deposit, in such a state of entire purity, in any 

 other region. I have met with no description of any such stra- 

 tum out of New Jersey, either in Europe or among the creta- 

 ceous masses of our Southern States. 



Beds of a dark blue tenacious clay, not unlike the gault of 

 England, occur sometimes associated with these beds of marl, 



