22 



PRINCIPLES OF PALAEONTOLOGY. 



of slices ground down to a thinness sufficient to render them 

 transparent ; but in the softer kinds the rock must be disinte- 

 grated under water, and the 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 im- 

 portant 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- 

 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, 

 Huxley, Wallich, and others, 

 it has, however, been shown 

 that there is now forming, in 

 the profound depths of our 

 great oceans, a deposit which 

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

 generally known as the " Atlantic ooze," 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 

 Globigerina, absolutely indistinguishable from the Gtobigerince. 

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

 these occur fragments of the skeletons of other larger creatures, 



Fig. 7. Section of Gravesend Chalk, 

 examined by transmitted light and highly 

 magnified. Besides the entire shells of 

 Globigerina, Rotalia, and Textularia, 

 numerous detached chambers of Globi- 

 gerina are seen. (Original.) 



