440 SECONDARY LIMESTONE. 



The dullness of the fracture may diminish, 

 while other circumstances continue the same; 

 and thus they acquire a more compact and a 

 harder aspect, with corresponding varieties of 

 surface on fracture. This appearance of com- 

 pactness is often so perfect, that the fractured 

 surface is as uniform and smooth as that of pitch 

 or glass ; and, in these cases, it is sometimes even, 

 at others, conchoidal, and, occasionally, splintery. 



The secondary limestones, like the primary, 

 are sometimes crystalline ; and that crystalline 

 texture presents as many modifications of appear- 

 ance in the one class, as it does in the other. To 

 detail these varieties could serve no purpose ; but 

 it may be added, that it is not uncommon, in 

 certain situations, for crystalline particles to be 

 intermixed in a base of which the prevailing cha- 

 racter is compact or earthy. 



These resemblances of character between 

 many limestones of the secondary and others of 

 the primary class, are of considerable import- 

 ance. They serve to prove that which has 

 already appeared in many other cases, namely, 

 that the mineral distinctions between the two, 



