48 OOLITIC AND ORGANIC LIMESTONES. 



their resemblance in structure to the eggs or hard roe of fish. The 

 grains are rarely -^th of an inch in diameter, are spheroidal, cemented 

 together with carbonate of lime, and when sliced and examined under 

 the microscope, are seen to consist of concentric layers of crystalline 

 calcite, arranged about a nucleus, which is sometimes a grain or two of 

 sand, but more frequently a minute foraminiferous shell. Occasionally 

 the grains are radiate, and sometimes they have recrystallised. This 

 structure is seen in some parts of the Bala limestone, and occasionally in 

 the Plymouth limestone, and parts of the Carboniferous limestone near 

 Bristol, &c. ; it is characteristic of much of the Inferior Oolite, Lincoln- 

 shire limestone, Bath oolite, Coralline oolite, and Portland oolite. Oolitic 

 grains occur in Tertiary freshwater limestones of the Isle of Wight. Von 

 Buch has mentioned a stalactitic layer of limestone, which is some- 

 times oolitic, covering the lavas of the island of Lancerote. It has 

 been suggested that the deposit is due to north-west winds in winter, 

 driving the spray of the sea over the island. A similar rock is de- 

 scribed by Mr. Darwin at St. Helena, where masses of white finely 

 oolitic rock are attached to the outside of some of the incrusted 

 pebbles ; and Mr. Sorby has described oolitic grains in the recent lime- 

 stones from Bahama and Bermudas. Concretions of a concentric 

 nature, but of far larger size and darker colour, are formed at Carlsbad 

 by the waste water of the hot mineral spring. In these recent instances 

 the grains may have formed in a subaerial way ; but in the case of the 

 great Secondary limestones, there can be no doubt that their formation 

 is due to evaporation of the surface of the sea, so that a film was formed 

 around some shell fragment, and continued to increase in size as it fell 

 through the water till it sank to the bottom. This explanation will 

 also account for the uniform size of the grains in the same stratum, 

 Sometimes beds occur, like the Pea Grit,at the base of the Inferior Oolite, 

 in which the grains are as large as peas, and often of irregular shape. 



Foraminiferal Limestone. Many limestones are composed chiefly 

 of the remains of animals, such as corals, foraminifera, shells, encri- 

 nites, &c. The chalk is the best example in this country of a fora- 

 miniferal limestone ; but the limestones formed by the Miliola, and 

 especially by the Nummulina, constitute great beds on the Continent, 

 and the latter ranges east and west through the central region of the 

 Old World. Deposits like the chalk are now forming at the bottom of 

 all the deep oceans, chiefly by the accumulation of foraminifera named 

 Globtgerinct and Orbulina, with a few pteropods which also live in the 

 surface waters, and sink to the bottom after death to become mixed 

 with sponges, sea-urchins, shells, and crustaceans, which live at great 

 depths. 



Coralline Limestone. Among the limestones in this country, 

 largely formed of corals, are the Wenlock limestone of the border coun- 

 ties of Wales, the Plymouth limestone of South Devon, parts of the 

 Carboniferous limestone, especially of Derbyshire, and the Coral 

 Rag. Probably all these corals lived in moderate depths, for there 

 is no evidence that they formed great coral reefs, such as exist at the 

 present day, and extend down to great depths in the ocean. Tho 



