24 INTRODUCTION. 



deposit is the so-called " Infusorial earth " of Kichmond in 

 Virginia (fig. 8), 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 sec- 

 tions, 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 a Sea- 

 urchin. As the flint has completely surrounded and infil- 

 trated 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. 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 carbonate 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 suitable methods of treatment, even in the actual matrix 

 or basis of the flint. 1 



In addition to deposits formed of flint itself, there are 

 other siliceous deposits formed by certain silicates, and also 



1 It has been asserted that the flints of the chalk are merely fossil sponges. 

 No explanation of the origin of flint, however, can be satisfactory, unless it 

 embraces the origin of chert in almost all great limestones from the Silurian 

 upwards, as well as the common phenomenon of the silicification of organic 

 bodies (such as corals and shells) which are known with certainty to have been 

 originally calcareous. It should also be mentioned that some of the flints of 

 the chalk are certainly only secondarily of organic origin, if even that. This 

 is the case with the tabular masses of flint filling cracks and joints in the 

 chalk. These masses were not produced contemporaneously with the chalk, 

 but have been formed at a later period by the percolation into fissures of the 

 rock of water holding silica in solution. 



