THE FOSSILIFEROUS ROCKS. 



debris examined microscopically. When investigated by either of 

 these methods, chalk is found to be a genuine organic rock, being 

 composed of the shells or hard parts of innumerable marine 

 animals of different kinds, some entire, some fragmentary, 

 cemented together by a matrix of very finely granular carbonate 

 of lime. Foremost amongst the animal remains which so largely 

 compose chalk are the shells of the minute creatures which will be 

 subsequently spoken of under the name of Foraminifera (fig. 7), 



and which, in spite of their 

 microscopic dimensions, play a 

 more important part in the 

 process of lime-making than 

 perhaps any other of the larger 

 inhabitants of the ocean. 



As chalk is found in beds 

 of hundreds of feet in thick- 



examined by transmitted light and highly 

 magnified. Besides the entire shells of 

 Gflobigerina, Rotalia, and Textularia 

 of 



ness, and of great purity, there 

 was long felt much difficulty 

 in satisfactorily accounting for 

 its mode of formation and ori- 

 gin. By the researches of 

 Carpenter, Wyville Thomson, 

 Fig. 7.-Section of Gravesend Chalk, Huxley, Wallich, and others, 



it has, however, been shown 



. .... 



that there IS now forming, in 



< he p r f und ****** 



great oceans, a deposit which 

 is in all essential respects identical with chalk, and which is 

 generally known as the " Atlantic ooz, " from its having been 

 first discovered in that sea. This ooze is found at great 

 depths (5000 to over 15,000 feet) in both the Atlantic and 

 Pacific, covering enormously large areas of the sea-bottom, 

 and it presents itself as a whitish-brown, sticky, impalpable mud, 

 very like, greyish chalk when dried. Chemical examination 

 shows that the ooze is composed almost wholly of carbonate of 

 lime, and microscopical examination proves it to be of organic 

 origin, and to be made up of the remains of living beings. 

 The principal forms of these belong to the Foraminifera, and 

 the commonest of these are the irregularly-chambered shells of 

 Globigcrina, absolutely indistinguishable from the Globigerina: 

 which are so largely present in the chalk (fig. 8). Along with 

 these occur fragments of the skeletons of other larger creatures, 

 and a certain proportion of the flinty cases of minute animal and 



