FORAMINIFERA. 



119 



tain the tests of Foraminifera. In the Carboniferous period, most 

 of the limestones can be shown by microscopical examination to 

 contain the shells of Foraminifera in larger or smaller numbers (see 

 fig. 4) ; and some of them are so largely composed of the cases of 

 these minute organisms as to become truly " Foraminiferal lime- 

 stones." Of this nature are the great Fusulina limestones of Russia 

 and North America, and the Saccammina limestone of Britain. It 

 is interesting to notice that the highly specialised genus Niimmidina, 

 which attained such a vast development in the Tertiary period, is 

 represented for the first time in the Carboniferous limestone. In 

 the Permian limestones Foraminifera are tolerably abundant, but on 

 the whole nearly resemble their Carboniferous predecessors. 



In the Mesozoic period, the remains of Foraminifera are more or 

 less abundant in the Triassic and Jurassic systems, occurring in 

 limestones, shales, or marls, and sometimes being present in sufficient 

 numbers to give rise to regular Foram- 

 iniferal limestones. In the Cretaceous 

 period, the most remarkable Foramini- 

 feral rock is the White Chalk itself. This 

 well-known and widely extended forma- 

 tion can be readily shown by micro- 

 scopical investigation to be very largely 

 made up of the entire or broken shells 

 of Foraminifera, amongst which the 

 genus Globigerina plays a predominant 

 part (fig. 28). There is thus a close 

 and striking similarity between the White 

 Chalk and the " Globigerina ooze " of 

 modern seas. The principal distinction 

 between Chalk and consolidated Globi- 

 gerina mud is found in the fact that the 

 latter contains a considerable proportion 

 of siliceous matter, while the former con- 

 sists of from ninety-five to ninety-eight 

 per cent of carbonate of lime, and is 

 nearly or quite free from disseminated silica. This distinction is, 

 however, more apparent than real, and depends in reality upon the 

 changes which the Chalk has undergone subsequent to its original 

 deposition. The siliceous matter in the modern Globigerina mud is 

 due to the presence of a larger or smaller intermixture of the flinty 

 tests of Radiolarians, the spicules of siliceous Sponges, the tests of 

 Diatoms, and the like. There is no reason to doubt, however, that 

 the Chalk, when first laid down, similarly contained a larger or 

 smaller quantity of siliceous matter in the form of the skeletons 

 of flint-producing organisms such as Sponges. The silica of these 



Fig. 28. — Section 



of White 

 Chalk, from Sussex, enlarged 

 about fifty times. (Original.) 



