448 SECONDARY LIMESTONE. 



This is found in Egg, and elsewhere, with 

 ordinary varieties of the former. 



c. Prismatic, parallel or radiating: the 

 prisms more or less easily separable : 

 sometimes striated, and with an ob- 

 scure appearance of joints: pseudo- 

 organic; Madreporite of mineralo- 



E. Concretionary spheroidal. Peastone, roe- 

 stone, and oolite. 



a. With large spherules, sometimes com- 



pressing each other. 



The structure of the spherule is often dis- 

 tinctly laminar arid it scales off' in concentric 

 crusts : rare ; in some cases, apparently of recent 

 alluvial origin. 



b. With spherules of a moderate size: 



roestone. 



e. With minute spherules, varying much 

 in size and sometimes becoming in- 

 distinct : oolites of England. 



O 



d. The spherules intermixed with frag- 

 ments of irregular forms. Purbeck 

 stone. 



F. Fragile, or easily sectile. 



a. With a smooth somewhat glossy frac- 



ture : indurated chalk. Ireland. 



b. With an earthy fracture ; compact, but 



soft : common chalk. 



