446 BLUE MARL. 



vegetable extract. This whole body of marl, both the dry and yel- 

 low lying at top, as well as the blue and wet below, was all brought 

 and deposited by currents, as is manifest by the different layers of 

 different specific gravity, and still more by the many intervening 

 layers of a fine calcareous clay (before mentioned), which may be 

 considered as the true marl of mineralogy, though in very small 

 quantity. Analyses were carefully made of every different quality, 

 and the results may be interesting as showing how much one layer 

 may vary from the one next adjoining; and different specimens not 

 more than a few inches of perpendicular distance apart. 



Upper dry part, yellow, and loose as sand, varying 

 (by unevenness of surface) from 3 to 7 feet, con- 

 tained of carbonate of lime . . . .53 per cent. 



Next layer below, brownish yellow, through which 



water passes, ...... 25 " 



About 12 inches lower, in the blue, . . .64 " 

 " " " " " " another specimen 

 below ........ 69 " 



Layers of clay marl, interspersed through the above 9 " 



And in a subsequent digging, the strength of four specimens of 

 the blue part of the marl was as follows : 



In the first foot depth of blue under-stratum . 32 per cent. 



In the second foot 33 " 



At 3 feet .... 76 



At 4 feet, and lowest digging then effected . 70 " 



It may readily be inferred, from these various results, that if 

 one or two specimens only had been analyzed, and these taken with 

 no more care than is commonly used, that a very deceptions report 

 would have been furnished from making even the most accurate 



Conchologists and geologists, who have treated so much of marls, 

 but merely in reference to the shells they furnish, or to their 

 geological character, speak of the blue marl as formed by shells 

 being imbedded in a blue day. But the earth is not generally a, 

 clay, nor anything even approaching to a clay, but is mostly of 

 silicious sand. The ordinary blue marl contains usually from three 

 to four times as much pure and separable silicious sand as of clay. 

 From various specimens of two diggings in such marl, from which 

 more than 300 acres were marled of the Coggins Point farm, the 

 following results were found by analysis : 



Yellow marl (wet) thin layer at top, contained of car- 

 bonate of lime ,,,, 24 grains. 



