IN THE MUD OF THE LEVANT. 103 



yet their arrangement and character have been so 

 altered that the spine, when fractured, breaks up, 

 not in the direction of the organization, but along 

 the lines of cleavage, characteristic of calcareous 

 spar, and that nothing is easier than to obtain 

 out of one of these spines a number of perfect rhom- 

 boidal crystals, which nevertheless exhibit, under 

 the microscope, all the interesting structure which 

 Dr. Carpenter has shewn to be peculiar to the 

 Echinodermata. Here is obviously an instance 

 of double action. The place of each atom, as it 

 was removed, must have been supplied by another 

 of the same substance, only it has been rend- 

 ered obedient in its re-deposition to the laws 

 which regulate crystallization, rather than those 

 of organic life. At the same time, why the 

 larger organisms should retain their original 

 structure and contour, whilst the microscopic 

 forms alone are replaced by pisolites, roestones, 

 and crystalline limestones, is a question that I am 

 unable fully to answer. Analogous phenomena, 

 however, exist elsewhere, which shew that an 

 extensive metamorphic action, either chemical or 

 volcanic, sufficiently powerful to destroy all the 

 smaller organisms, does not of necessity affect 

 the integrity of the larger fossils. Sir R. I. Mur- 

 chison has recently shewn us that very extensive 



