THE FOSSILIFEROUS ROCKS. 35 



schiefer") of Bohemia, largely worked as polishing-powder, is 

 composed wholly, or almost wholly, of the flinty cases of Diatoms, 

 of which it is calculated that no less than forty-one thousand 

 millions go to make up a single cubic inch of the stone. Another 

 celebrated deposit is the so-called " Infusorial earth " of Rich- 

 mond in Virginia, where there is a stratum in places thirty feet 

 thick, composed almost entirely of the microscopic shells of 

 Diatoms. 



Nodules or layers of flint, or the impure variety of flint 

 known as chert, are found in limestones of almost all ages from 

 the Silurian upwards; but they are especially abundant in the 

 chalk. When these flints are examined in thin and transparent 

 slices under the microscope, or in polished sections, they are 

 found to contain an abundance of minute organic bodies such 

 as Foraminifera, sponge-spicules, &c. embedded in a siliceous 

 basis. In many instances the flint contains larger organisms 

 such as a Sponge or Sea-urchin. As the flint has completely 

 surrounded and infiltrated the fossils which it contains, it is 

 obvious that it must have been deposited from sea-water in a 

 gelatinous condition, and subsequently have hardened. N That 

 silica is capable of assuming this viscous and soluble condition 

 is known; and the formation of flint may therefore be regarded 

 as due to the separation of silica from the sea-water and its 

 deposition round some organic body in a state of chemical 

 change or decay, just as nodules of phosphate of lime or carbon- 

 ate of iron are produced. The existence of numerous organic 

 bodies in flint has long been known ; but it should be added that 

 a recent observer (Mr. Hawkins Johnson) asserts that the 

 existence of an organic structure can be demonstrated by suit- 

 able methods of treatment, even in the actual matrix or basis of 

 the flint. * 



In addition to the deposits formed of flint itself, there are 

 other siliceous deposits formed by certain silicates, and also of 

 organic origin. It has been shown, namely by observations 

 carried out in our present seas that the shells of Foraminifera 

 are liable to become completely infiltrated by silicates (such as 

 " glauconite, " or silicate of iron and potash). Should the actual 

 calcareous shell become dissolved away subsequent to this infil- 

 trationas is also liable to occur then, in place of the shells of 



* It has been asserted that the flints of the chalk are merely fossil 

 sponges. No explanation of the origin of flint, however, can be satisfac- 

 tory, unless it embraces the origin of chert in almost all great limestones 

 from the Silurian upwards, as well as the common phenomenon of the 

 silincation of organic bodies (such as corals and shells) which are known 

 with certainty to have been originally calcareous. 



