THE WORK OF ORGANISMS IN ROCKS. 9 



Phosphate of Lime, Grlauconite, Silica, Red Clay, Ferru- 

 ginous and Carbonaceous material. Some of these, such 

 as Glauconite and Red Clay, are not secreted directly in 

 the form in which we find them, but are the products of 

 changes which the secretions have undergone. 



Carbonate of Lime is the most important of all the 

 materials excreted, and the " lime habit " may be found 

 in every branch of the animal kingdom, as well as in 

 many plants. 



Marine Algae (Nullipores, Halimeda, Cymopolia, &c.) 

 are important reef builders, while on land limestones are 

 found composed largely of Chara. 



Even in hot springs such as those of Iceland, the 

 Yellowstone Park, Carlsbad, the Azores, &c, it has been 

 found that vegetable life exists, and the travertine and 

 tufa formed in connection with these springs is now attri- 

 buted to the action of bacteria, algae and other lowly plants. 



In Foraminifera, Sponges, Corals, Spirorbis, Poly- 

 zoa, Estheria, Barnacles, Brachiopods, Lamellibranchs, 

 Gasteropods, Pteropods, Cephalopods, Echinoderms, all lime- 

 forming animals, we have nearly all of the great divisions of 

 the Invertebrata represented, and most vertebrates have 

 carbonate and phosphate of lime in their bony tissues. It 

 would be too long a task to describe the characteristics 

 and extent of the vast deposits of ooze, chalk, limestone, 

 coral reefs, bone beds, &c, formed by these means, and 

 our object is to discuss the question how the organism 

 works rather than what is produced. 



Silica is secreted principally by Diatoms and Radio- 

 laria, and siliceous oozes formed of these lowly plants and 

 animals cover great areas of the ocean floor. Spong-es and 

 other groups have siliceous spicules in their tissues, and 

 these are the main factors in the production of Plint and 

 Chert. 



