22 INTRODUCTION. 



iniferal limestones of the Mesozoic period the most interesting and 

 important is the White Chalk, the characters of which will be con- 

 sidered more fully later on. Lastly, in the Tertiary period there 

 are various well-known Foraminiferal limestones, of which the most 

 important and most widely distributed is the great calcareous 

 deposit known as the " Nummulitic limestone." 



Among Ccelenterate animals, the principal lime-makers are the 

 true Corals (Madreporarid). Many of the limestones which enter 

 into the composition of the crust of the earth, dating from the 

 Ordovician period onwards, are more or less extensively composed 

 of the skeletons of Corals. Though the Corals commonly occur in 

 such a manner in the rock as to show clearly that they grew on the 

 spot where the limestone was formed, it is questionable if any of the 

 Palaeozoic limestones can be properly said to be actual " coral- 

 reefs," though some of the Devonian limestones of North America 

 and Belgium may possibly be truly of this nature. In the Secondary 

 and Tertiary periods, however, we meet with coralline limestones 

 which may be considered as essentially similar in structure and mode 

 of formation to the " coral-reefs " of the warm seas of the present 

 epoch. The Corals, however, are not the only Ccelenterate ani- 

 mals which play an important part in the formation of limestones, 

 for it is now known that certain of the Hydrozoa are likewise capable 

 of giving rise to extensive calcareous deposits by the accumulation 

 of their skeletons. Thus, certain of the Silurian and Devonian 

 limestones are largely composed of the calcareous skeletons of the 

 extinct Hydrozoa which constitute the group of the " Stromatopo- 

 roids." Other Palaeozoic limestones are extensively made up of the 

 remains of organisms like Solenopora and Mitcheldeania, which are 

 possibly referable to the Hydrozoa. At the present day, the only 

 Hydrozoon which is conspicuously concerned in the formation of 

 limestone is the Hydrocoralline genus Millepora, which plays an 

 important part in the construction of many of the existing " coral- 

 reefs." 



Of the Echinodermata there is only one order — viz., that of the 

 Sea-lilies or Crinoids — which demands special mention in the pres- 

 ent connection. At the present day, the Crinoids constitute a but 

 feeble remnant of a once powerful and widely distributed group, and 

 they are not known to exist anywhere in numbers sufficient to render 

 them noteworthy as lime-makers. Among the older rocks of the 

 earth's crust, however, and more particularly in deposits of Ordovi- 

 cian, Silurian, Devonian, and Carboniferous age, are found great 

 beds of limestone, essentially composed of the broken stems and 

 detached plates of Crinoids (fig. 7). Such limestones are known to 

 geologists as " Crinoidal limestones " and " Encrinital marbles," 

 and they are usually composed of more or less broken and rolled 



