MINUTE ORGANISMS AS ROCK-MAKERS 



I08; 



to the decomposing organic matter, becomes more pronounced, while 

 perfect shells of Globigerina almost disappear, fragments become 

 smaller, and calcareous mud, structureless, and in a fine state of 

 division, is in greatly preponderating proportion. One can have no 

 doubt, on examining this sediment, that it is formed in the main by 

 the accumulation and disintegration of the shells of Globigerina ; the 

 shells fresh, whole, and living in the surface-layer of the deposit ; 

 and in the lower layers dead, and gradually crumbling down by the 

 decomposition of their organic cement, and by the pressure of the 

 layers above.' This white calcareous mud also contains in large 

 amount the 'coccoliths' and ' coccospheres ' formerly mentioned. 

 Now the resemblance which this Globiyerina-m\.u\. when, dried, bears 



FIG. 811. Microscopic organisms in chalk from Gravesend : a, b, c, d, 

 Texinlaria . globulosa : <, e, e, h'otnliii aspera : f, Textularia <///, ;,ta; 



ifi l/<:ras; ) 



to chalk is so close as at oner to suggest the similar origin of the 

 lattev ; and this is fully confirmed by microscopic examination. For 

 many samples of it consist in great part of the minuter kinds of 

 Foraminitera. especially Globigerince, whose shells are imbedded in a 

 mass of apparently amorphous particles, many of which, nevertheless, 

 present indications of being the disintegrated fragments of similar 

 shells, or of larger calcareous organisms. In the chalk of some 

 localities the disintegrated prisms of Pinna, or of other large shells 

 of the like structure (as fnoceramus), form the great bulk of the 

 recognisable components ; whilst in other cases, again, the chief part 

 is made up of the shells of f 'i/t/n-rina. a marine form of entomo- 

 stracous crustacean. Different specimens of chalk vary greatly in 



