202 A LUMP OP CHALK AND ITS LESSONS. 



shells, wliile others are 2>ortioiis of the spines of sea- 

 urchins, and others, again, are the flinty spicules of 

 sponges. By far the larger proportion consists, however, 

 of jsei'fect objects of extremely minute size, mainly be- 

 longing to that lovely group of animals known as 

 foraminifera, or shortly, " forams." Of these beautiful 

 little shells some are coiled in a manner recalling the shell 

 of the nautilus, while others consist of globular masses 

 arranged either in a coil or in a straight line, the globules 

 gradually nicreasing in size from the summit to the mouth 

 of the shell. In all cases, however, the walls of these 

 shells are perforated by the inconceivably minute ai^ertures 

 from which the forams take their name, and through 

 which, when alive, the creatures jJroti-uded delicate threads 

 of the jelly-like protoplasm of which their soft parts are 

 composed. Truly marvellous in beauty are these forams, 

 although pages of description can give but a faint idea of 

 them, and the student should see them for himself under a 

 microscope. So numerous, however, are these forams, 

 and other equally minute organisms in the white chalk, 

 that they f recjuently compose half its substance, while it is 

 stated that in some rare cases they may even rise to as 

 much as ninety per cent. 



We have now, therefore, to add to our definition of chalk 

 that it is largely composed of the shells of the minute 

 animals kno^'n as forams, together with those of other 

 allied creatures, and so many accordingly speak of it as a 

 limestone which is evidently, to a large extent, of organic 

 origin. Moreover, as these forams are more or less closely 

 allied to species inhabiting the ocean at the present day, 

 we should be justified from this evidence alone in regard- 

 ing the chalk as a formation of marine origin. This orio-in 

 is, however, equally well jjroved by the larger fossils, such 

 as the shells of sea-urchins, scallops, oysters, &c., commonly 

 occurring in the chalk ; while, in addition to this, the 

 extreme jiurity and thickness of the formation would of 

 itself be sufficient to demonstrate that the chalk is the 

 result of long-continued deiiositioii on the Ijottom of the 

 sea. 



Thus much for the composition of our lump of chalk as 



