OLDER BRITISH LIMESTONES. 109 



pieces when the animal matter disappears, but the separate joints or 

 plates are rarely minute ; and sometimes, as in the mountain limestone, 

 form a rock in which the eye can easily distinguish the component 

 materials by the calcite cleavage. 



Several recent limestones, like those of Bahama and Bermuda, 

 contain oolitic grains. They have the aspect of having been formed in 

 water containing mud derived from corals and decayed shells, but 

 they are essentially chemical deposits, and show a granular texture in 

 some grains and a crystalline texture in others. Thus, all the types 

 of limestone which are met with in the strata are recognised as being 

 in process of formation at the present day, and the following short 

 account of the limestones as observed under the microscope by Dr. 

 Sorby l will sufficiently demonstrate the conditions under which they 

 severally came into existence. 



The Bala Limestone is one of the oldest of British limestones. As 

 typically seen, it consists almost wholly of entire joints of an encrinite 

 embedded in fine-grained material which is for the most part made up 

 of mica or chlorite, such as is found in the associated slates. Locally, 

 it is oolitic. 



The Wenlock Limestone is chiefly formed of more or less com- 

 minuted encrinites, with fragments of corals, bryozoa, brachiopods, 

 and trilobites. Near Malvern, oolitic grains are sometimes found 

 in this rock ; and in the finer-grained beds Entomostraca, are 

 common. 



The Aymestry Limestone and calcareous beds in the Ludlow 

 rocks are essentially similar in composition, except that fragments of 

 brachiopod shells are more abundant, and there are occasionally por- 

 tions of bone, probably fish. 



The Devonian Limestones, seen at Ilfracombe, Torquay, and 

 Plymouth, are chiefly composed of joints of encrinites and fragments 

 of coral. The fragments of brachiopod and other shells are much rarer. 

 As in the Silurian limestones, foraminifera are not met with. Some- 

 times the Devonian limestones are converted into dolomite. 



The Carboniferous Limestone. In the typical localities the greater 

 part of this rock consists of joints of encrinites, sometimes entire, 

 sometimes broken, associated with fragments of brachiopoda and 

 foraminifera, which are often as abundant as in average specimens of 

 chalk. Recognisable fragments of corals and polyzoa are found, and 

 sometimes shell prisms occur in great quantity. Occasionally copro- 

 lites, teeth, and fragments of bone form not unimportant consti- 

 tuents of the rock. At Bristol and other places some of the beds are 

 oolitic. In Derbyshire and elsewhere some beds are changed into 

 almost pure dolomite, but the organisms give no proof that the rock 

 consisted originally of magnesia to any appreciable extent. 



Magnesian Limestone. Many beds originally contained oolitic 



grains and shell fragments ; and such beds have exactly the structure 



that would be developed in an oolitic rock, if it became crystalline, 



by having half of its lime replaced by magnesia. The bulk of the 



1 Address, Quart. Jour. Geol. Soc., vol. xxxv. p. 77, &c. 



