THE DAWN-ANIMAL AS A TEACHEK IN SCIENCE. 219 



and if its life-processes are just as inscrutable as those 

 of higher creatures, we can at least inquire as to its 

 history in geological time. In this respect we find in 

 the first place that the Protozoa have not had a monopoly 

 in their profession of accumulators of calcareous rock. 

 Originated by Eozoon in the old Laurentian time, 

 this process has been proceeding throughout the geo- 

 logical ages; and while Protozoa, equally simple with 

 the great prototype of the race, have been and are 

 continuing its function, and producing new limestones 

 in every geological period, and so adding to the 

 volume of the successive formations, new workers of 

 higher grades have been introduced, capable of enjoy- 

 ing higher forms of animal activity, and equally of 

 labouring at the great task of continent-building ; of 

 existing, too, in seas less rich in mineral substances 

 than those <of the Eozoic time, and for that very reason 

 better suited to higher and more skilled artists. It is 

 to be observed in connection with this, that as the work 

 of the Foraminifers has thus been assumed by others, 

 their size and importance have diminished, and the 

 grander forms of more recent times have some of them 

 been fain to build up their hard parts of cemented sand 

 instead of limestone. 



But we further find that, while the first though 

 not the only organic gatherers of limestone from the 

 ocean waters, they have had to do, not merely with 

 the formation of calcareous sediments, but also with 

 that of silicious deposits. The greenish silicate called 

 glauconite, or greensand, is found to be associated 



