THE FOSSILIFEROUS ROCKS. 25 



florin. There are, however, as we shall see, many other lime- 

 stones, which are likewise largely made up of Foramitiiferay 



Fig. lo. — Piece of Nummulitic Limestone from the Great Pyramid. 

 Of the natural size. (Original.) 



but in which the shells are very much more minute, and would 

 hardly be seen at all without the microscope. 



We may, in fact, consider that the great agents in the pro- 

 duction of limestones in past ages have been animals belonging 

 to the Crinoids, the Corals, and the Foraininifera. At the pre- 

 sent day, the Crinoids have been nearly extinguished, and the 

 few known survivors seem to have retired to great depths in 

 the ocean ; but the two latter still actively carry on the work 

 of lime-making, the former being very largely helped in their 

 operations by certain lime- producing marine plants (Nullipores 

 and Corallines). We have to remember, however, that though 

 the limestones, both ancient and modern, that we have just 

 spoken of, are truly organic, they are not necessarily formed 

 out of the remains of animals which actually lived on the 

 precise spot where we now find the limestone itself. We may 

 find a crinoidal limestone, which we can show to have been 

 actually formed by the successive growth of generations of 

 sea-liHes in place ; but we shall find many others in which the 

 rock is made up of innumerable fragments of the skeletons of 

 these creatures, which have been clearly worn and rubbed by 

 the sea-waves, and which have been mechanically transported 

 to their present site. In the same way, a limestone may be 

 shown to have been an actual coral-reef, by the fact that we 

 find in it great masses of coral, growing in their natural posi- 



