THE WORK OF ORGANISMS IN ROCKS. 13 



two forms, calcite and aragonite. Some shells and tests 

 are composed exclusively of the former, others of the 

 latter, and some show both in the same shell either in 

 layers or in a more intimate admixture. 



The elementary laws of evolution would lead us to 

 expect that some forms would find the spicules or shells of 

 advantage for support or protection against enemies, and 

 the " habit " would become essential for the continuance 

 of the species. 



Applying the principle generally to organisms which 

 secrete lime, silica, iron or other substances out of 

 solution in water, whether in fresh or salt water, we find 

 that unicellular organisms such as Foraminifera, Radio - 

 laria and Diatoms form tests or coverings following 

 roughly the outlines of the cell. In Sponges we find 

 spicules of lime or silica, providing support or protection 

 by means of skeletal and dermal spicules. Some Corals, 

 Echinoderms and Polyzoa have spicules, while others 

 secrete a continuous covering round the polype. Molluscs 

 advance a stage further and set apart portions of the 

 mantle or other organ to secrete shells, and in higher 

 forms we may look upon the ossification of the notochord 

 and other structures as the result of similar processes. 



After the work of deposition has been completed 

 organisms may still exert a further influence in cementing 

 the loose material into a compact rock. In this their 

 action is not confined to organic deposits, but loose detritai 

 matter, such as sand and fragmentary particles, may be 

 taken and used to form a covering. Thus Sabellaria, our 

 only British reef-forming animal, forms reefs round our 

 coasts. In Central France the ancient fresh-water lake of 

 Limagne is largely filled up by a limestone formed of shell 

 fragments and other loose pieces cemented by Phryganea. 

 Ant hills too are composed of grains agglutinated by secre- 



B 



