SECONDARY FORMS. 87 



Thus the dodecahedral variety of carbonate of 

 lime, commonly called dog-tooth spar, occurs the 

 most frequently in Derbyshire. 



In Cumberland, the most common variety is a six- 

 sided prism terminated by the planes of an obtuse 

 secondary rhomboid. 



In the Hartz, the entire six-sided prism occurs 

 more frequently than in other places. 



Particular secondary forms are found to occur con- 

 stantly among some species of minerals, and rarely 

 among other species belonging to the same class of 

 primary forms. 



Thus the regular hexagonal pyramids, which occur 

 constantly among the secondary forms of quartz, 

 rarely occur in carbonate of lime. 



The causes of these peculiar habitudes of minerals 

 have not I believe been investigated, nor do I appre- 

 hend that the investigation would lead to any satis- 

 factory result. They appear to belong to that class 

 of facts, which our limited knowledge of the opera- 

 tions of nature does not enable us at present to com- 

 prehend. 



