SECONDARY LIMESTONE. 437 



rarely, and it presents several varieties of appear- 

 ance. In some cases, the prisms are simple and 

 parallel ; in others, simply radiated from one 

 center; or else they interfere with each other as 

 they proceed from various centers. These prisms 

 are sometimes, but not necessarily, crystalline ; 

 and they are frequently imbedded, in the form 

 of distinct radiating- concretions, in continuous 

 masses of an earthy character. In a few rare 

 instances they are not only striated longitudinally, 

 but marked at intervals by protuberant joints ; a 

 structure formerly mentioned as also occurring 

 in some ironstones and jaspers. From the re- 

 semblance which some of the above-mentioned 

 appearances present to organic bodies, these va^ 

 rieties have been distinguished by the name of 

 madreporite. 



The spheroidal structure occurs in limestones 

 under three very distinct forms. 



In the first of these, spherical grains of lime- 

 stone are agglutinated into a solid mass, the par- 

 ticles varying from the size of poppy seed to that 

 of a pea ; and these rocks are known by the name 

 of oolites. The grains are sometimes simple; 



