How Chalk is Made 



same kind of sheltering skeletons. Some con- 

 struct delicate domes or many-chambered discs 

 of sand-grains, joined together with enough 

 carbonate -of- lime to act as building -cement. 

 Such skeletons are called '* tests," to distinguish 

 them from "shells" proper. 



Hardly anything can be more remarkable than 

 the way in which these tests are put together 

 by mere specks of jelly, alive indeed, but without 

 parts, without development, without understand- 

 ing. How and why they should choose from 

 one place, each the especial materials which go 

 to form its owm kind of '' test," is one of the 

 mysteries in Nature for which Science has no 

 explanation. We can but look and marvel. 



One kind of jelly-speck will use the larger 

 grains of quartz, arranging them, and joining 

 them Into a bottle-shaped test or shelter. 



Another, in the same spot, belonging to a 

 different species, will select tinier grains of the 

 same substance, and will build out of them a 

 rounded sphere, exquisitely modelled, with tiny 

 holes at Intervals for the protruding "limbs." 



Another, of yet a different species, picks out 

 the minutest of sand-o-ralns and of bits of 

 sponge-splcules, and knits them together, with- 

 out any cement, into delicate white globes, "like 



L 145 



