CRTNOTDAL LIMESTONE 449 



animals, whose remains would thus become incorporated with the 

 Lucina. 



Other Examples. On the modern coast we sometimes find 

 colonies of Mytihis cdulis growing in isolated spots, surrounded by 

 black mud deposits. As the colony increases the young individuals 

 will attach themselves to the older shells, remaining chiefly within 

 the circumscribed area occupied by the original colony, since any 

 transgression outside this area would be checked by the soft mud, 

 which offers no means of attachment. In this manner, on a slowly 

 subsiding bottom, a core of shell limestone may be formed, sur- 

 rounded by mud, in which only occasional shells are found. 



Some of the "Reef Knolls" described by Tiddeman (85) from 

 the Carbonic limestones of the Clitheroe district, England, appear 

 to be due to colonies of shell-building animals, more than to the 

 growth of reef corals. Their structure, however, is more like the 

 coral reefs than the tepee butte cores. 



BEDDED ZOOGENIC DEPOSITS. 



Crinoidal Limestones. Among the limestones of all geologic 

 formations, beds composed largely or wholly of crinoid remains are 

 common. These remains are generally the stem joints, and they 

 vary greatly in size, from that of a pin head or smaller to those 

 over one inch or more in diameter. Generally the fragments of each 

 bed are more or less uniform in size, this latter depending on the 

 species of crinoid which has flourished- in that region. Crinoids 

 are admirably adapted to the formation of limestone beds, because, 

 on the death of the animal, the long stems, as well as the calices and 

 arms, readily separate into their component ossicles, and thus with 

 little rearrangement furnish the material in proper size of grain for 

 a limestone bed. Kirk (56) has recently brought forward much 

 evidence to show that many crinoids separate from their anchorage 

 and become more or less planktonic in later life. Thus schools of 

 floating crinoids may be washed into shallow water, there to accu- 

 mulate as crinoidal limestones. The crinoidal character of an an- 

 cient limestone is not always noticeable on the fresh surface, but 

 on weathering this structure is brought out. When accumulations 

 occur within the zone of wave activity, cross-bedding and other 

 shallow water structures may result. 



Shell Limestones. Beds of unbroken shells or coquina are 

 likewise found in different geologic horizons. Often great beds of 

 limestone occur, in which one species of siiell predominates to the 



