34 



INTRODUCTION. 



called "Infusorial earth" of Richmond in Virginia (fig. 13), where 

 there is a stratum, in places thirty feet thick, composed almost 

 entirely of the microscopic shells of Diatoms. 



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

 siliceous deposits formed by certain silicates, and also of organic 



Fig. 12. — Shells of Polycystina from Fig. 13. — Cases of Diatoms in the Rich- 



" Barbados earth"; greatly magnified. mond "Infusorial earth"; highly magni- 

 (Original.) fied. (Original.) 



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 Fora?7iinifera, 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 by Dr 

 W. B. Carpenter 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 often composed in 

 part of the internal casts of the shells of Foraminifera ; and we have 

 thus another and a very interesting example how rock-deposits of 

 considerable extent and of geological importance can be built up by 

 the operation of the minutest living beings. 



Carbonaceous Deposits. — It only remains in connection with the 

 general subject of the organically formed rocks to shortly consider 

 the rock-deposits in which carbon is found to be present in greater 

 or less quantity. In the great majority of cases where rocks are 

 found to contain carbon or carbonaceous matter, it can be stated 

 with certainty that this substance is of organic origin, though it is 



