THE FOSSILIFEROUS ROCKS. 25 
florin. There are, however, as we shall see, many other lime- 
stones, which are likewise largely made up of Foraminifera, 
Fig. 1o.—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 Cvinoids, the Corads, and the Foraminifera. 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 (JVud/ipores 
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-lilies 2 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- 
