534 P1TCHSTONB. 



f. With a mixed fracture, the large con- 



choidal presenting a minute addition- 

 ally conchoidal surface. 



g. The conchoidal fracture covered with 



minute scales, so as to give, on a su- 

 perficial view, a porphyritic aspect. 



B. Concretionary. 



a. Flat, lamellar: the lamellae thick or 



thin ; it even becomes schistose, or 

 papery, or fine scaly. 



b. Curved, lamellar. 



c. Prismatic : the prisms simple, or jointed ; 



with plane, or with curved surfaces. 



d. Columnar, or cylindroidal ; similarly, 



either simple or jointed: the concre- 

 tion is also sometimes partly colum- 

 nar, and partly prismatic ; or has 

 plane and curved sides both. 



e. Spheroidal ; large. This is generally 



combined with the columnar. The 

 small spheroidal concretionary, is ar- 

 ranged with the pearlstones. 



f. Very imperfectly spheroidal concretion- 



ary: the transition into pearlstone. 

 It sometimes also passes into the por- 

 phyritic. 



Sometimes different structures are intermixed 

 in the same rock, as the columnar and prismatic 

 with the lamellar. 



