502 



FLINTS IN CHALK. 



erthan the upper chalk, and is sometimes used for building stone. 

 In France, the beds of chalk seldom attain the thickness which they 

 have in England. The French divide the chalk formation into the 

 lowest or chalk marl, with green particles, craie chloriiee^ or glauco- 

 nie crayeuse ; the middle or coarse chalk is of a greyish colour, and 

 intermixed with sand ; it contains whitish chert [craie grossiere, or 

 craie tvfeau) ; the upper or white chalk {craie blanche), which con- 

 tains nodules of common flint. 



M. Humboldt, after noticing the great intermixture of the sandy 

 calcareous and argillaceous beds, in the formations below chalk, and 

 which is greatly increased in the tertiary strata above chalk, observes, 

 " that nature seems to have relented in her tendency to form com- 

 plex mixtures, when chalk was deposited." In the chalk formation, 

 we find a vast assemblage of calcareous strata, composed of carbo- 

 nate of lime, with very litde intermixture of the other earths, and with- 

 out any alternation with argillaceous or siliceous strata. Chalk is 

 not, however, absolutely pure ; for, beside the nodules and veins of 

 flint that occur in it, but which bear no sensible proportion to the 

 whole mass, some of the strata contain an intermixture with siliceous 

 sand, and in other strata, calcareous earth is combined with magne- 

 sia. In some of the chalk strata in France, the magnesia exceeds 

 ten per cent., and, I believe, many of the English chalk strata con- 

 tain as great a proportion of magnesian earth. 



Chalk which contains a notable portion of magnesia, may gener- 

 ally be known by an appearance of dendritical spotted delineations 

 on the surface of the natural partings, and by minute black spots, 

 like grains of gunpowder, in the substance of the chalk. 



In chalk, the stratification is seldom so distinct as it is in many 

 other calcareous formations : this may be owing partly to the softness 

 of the beds, which appear to have yielded to pressure ; and to the 

 same cause we may probably ascribe the fractured state of the no- 

 dules of flint in chalk, which often appear whole, when they are im- 

 bedded in the rock, but when taken out, are found to be shivered 

 into innumerable angular fragments. The nodules of flint in the 

 chalk are commonly arranged, in pretty regular layers ; they occur 

 in detached concretions of various shapes and sizes : some of them 

 are believed to be the casts of spongiform zoophytes, and this is ren- 

 dered more probable by the frequent occurrence of fossil echini in 

 chalk, in which the internal part is filled with flint, and forms a per- 

 fect cast of the animal. In some of the chalk flints near Paris, there 

 are beautiful small crystals of sulphate of strontian. 



The constant occurrence of flint in the upper chalk, and the ap- 

 parent conversion of animal remains into flint, formerly gave rise to 

 much speculation respecting the origin of flint ; and it was at one 

 time maintained, that flint and chalk were convertible or capable of 

 undergoing a mutual transmutation : but whatever hidden processes 

 there may be in the great laboratory of the earth, by which all min- 



