164 



varieties. Even there, its use is very limited ; 

 as, in a very short space, or in a mass of very 

 in considerable extent, the structure may be both 

 simple and compound. It thus becomes inca- 

 pable of being applied even to the specimens 

 in a cabinet ; much less is it fitted to form the 

 basis of an arrangement. 



There are however a few rocks* such as lime- 

 stone, characterized by an absolute simplicity of 

 composition, or consisting of one acknowledged 

 mineral species. Others, like clay slate, present 

 a comparative degree of simplicity ; possessing 

 an uniform composition, and having been 

 ranked, although not very properly, among 

 mineral species. The frequent deviations of 

 these from the simple standard, serves to prove 

 that the integrant earths are rather in a state of 

 mixture than in that combination which is pro- 

 bably essential to a mineral species. If Mine- 

 ralogists have frequently enumerated such rocks 

 in their catalogues of minerals, the practice, 

 though inaccurate, is attended with some con- 

 venience ; as, in such a case, it is preferable to 

 err by excess than by defect. That enumeration 

 has v however, in some instances, been unwarrant- 



