CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 



15 



tolerably hard and compact ; but it never assumes the crys- 

 talline aspect and stony density of limestone, except it be in 

 immediate contact with some mass of igneous rock. By 

 mean.s of the microscope, the true nature and mode of for- 

 mation of chalk can be determined with the greatest ease. 

 In the case of the harder varieties, the examination can be 

 conducted by means of slices ground down to a thinness suf- 

 ficient to render them transparent ; but in the softer kinds 

 the rock must be disintegrated 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 frag- 

 mentary, 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 sub- 

 sequently spoken of under the 

 name of Foraminifera (fig. 2), 

 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 thickness, 

 and of great purity, there was long felt much difficulty in 

 satisfactorily accounting for its mode of formation and origin. 

 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 At- 



Fig. 2. Section of Gravesend Chalk, 

 examined by transmitted light and highly 

 magnified. Besides the entire shells of 

 Globigerina, Rotalia, and Textulnria, 

 numerous detached chambers of Globi- 

 gerina are seen. (Original.) 



