108 UPPER SECONDARY ROCKS. 



a mutual transmutation. Flint is nearly a pure si- 

 licious earth ; and such earth occurs, under different 

 forms, among nearly all the calcareous rocks, in a 

 greater or less proportion. 



Thus we find silicious earth intermixed with prim- 

 itive limestone ; we find rock crystals (quartz) im- 

 bedded in it ; and we have already stated that sili- 

 cious grains are often mixed with magnesian lime- 

 stone and the oolites. It is therefore not at all sur- 

 prising to meet with silicious earth (flint) in chalk. 

 So also y in lava, we find nodules of chalcedony ; and 

 chalcedony and flint are virtually the same, silica 

 forming much the largest proportion of both. It is 

 also to be recollected that the hardest rocks and 

 stones are permeable to water ; and flint, when first 

 taken out of the chalk, is found full of moisture 

 when fractured. Chalk is rarely distinctly stratified. 

 This is supposed to be owing to the softness of the 

 beds, which appear to have yielded to pressure. 

 Chalk-beds often contain magnesia; some of the 

 French chalk contains at least 10 per cent, of this 

 mineral. The organic remains of the chalk forma- 

 tion we have stated to be exclusively marine. With 

 many species of shells and remains of zoophytes, 

 the teeth, scales, and palates of fishes also occur; 

 and the great preservation in which they are found 

 renders it very probable that chalk was deposited 

 in a deep and tranquil sea. 



The vegetable remains in chalk are few, and ap- 

 pear to belong to the order otfuci ; lignite, however, 

 has been found in the lower bed of chalk, near Ro- 

 chelle, which Brogniart thinks may have been form- 

 ed of peat. 



