34 PRINCIPLES OF PAL/EONTOLOGY. 



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 trans- 

 parent 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 a 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. 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 phos- 

 phate of lime or carbonate of iron are produced. The exist- 

 ence 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.* 



In addition to 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 infiltration as is also liable to occur then, in place of 

 the shells of the Foraminifera, we get a corresponding number 

 of green sandy grains of glauconite, each grain being the cast 

 of a single shell. It has thus been shown that the green sand 

 found covering the sea-bottom in certain localities (as found 

 by the Challenger expedition along the line of the Agulhas 

 current) is really organic, and is composed of casts of the 

 shells of Foraminifera. Long before these observations had 

 been made, it had been shown by Professor Ehrenberg that 

 the green sands of various geological formations are composed 

 mainly of the internal casts of the shells of Foraminifera ; and 



* 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 

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

 with certainty to have been originally calcareous. 



