Protozoa. 



299 



tons slowly settle to the bottom. Dredgings from the 

 bottom of the Atlantic Ocean contain a gray mud which, 

 under the microscope, is found to be largely composed of 

 these shells. So where we find chalk rock on land, we 

 know that region was once the bed of the ocean. This 

 once soft mud has become hardened, by drying or by 

 pressure, sometimes by both, into hard rock, a variety of 

 limestone known as chalk. A generation ago the car- 



FIG. 174. A SHELL-BEARING PROTOZOAN (GLOBIGERINA). 



From Packard. 



penter and the schoolboy used a rough broken lump of 

 the chalk rock. The ordinary school crayon, however, 

 usually contains no chalk. Any one who has used the 

 old lump chalk will recall that occasionally he struck a 

 hard, flinty place, due to the mixture of other material. 



Silicious Earth. Other kinds of protozoans form their 

 shells of silica. Beds of this material are found in various 

 parts of the world and are used as polishing material, 

 under the names Tripoli, Barbadoes earth, etc. Many of 

 the shells of silica are exceedingly beautiful in form. 



