MINERALOGICAL ARRANGEMENT. 443 



the skill of modern chemistry, we are yet unable to 

 determine which of their component parts are essen- 

 tial to the composition of the substance analysed, 

 and which are but accidental mixtures. In the in- 

 stance of the crystallised sandstone from Fontain- 

 bleau, no doubt can be entertained that either the 

 carbonate of lime, or the grains of sand, must be 

 regarded as accidental mixture, and foreign to the 

 constitution of the species, accordingly as we chuse to 

 consider the specimens, as arenaceous quartz agglu- 

 tinated bj/ carbonate of lime, or as carbonate of lime 

 inclosing grains of quartz. 



It would, however, present little difficulty to the 

 chemist, to determine that the silex and lime are not 

 chemically combined in the sandstone : but there are 

 numerous other instances, in which even the sagacity 

 of a Berzelius has probably failed in discriminating 

 the matter accidentally present in several of the 

 species of minerals which have been analysed, from 

 that which is essential to the composition of each 

 particular species. 



These doub'ts are suggested by the observed fact, 

 that the crystalline form of the Fontainbleau sand- 

 stone is similar to one of the secondary forms of car- 

 bonate of lime ; and from remarking, that among the 

 minerals which chemical analysis would raise into 

 distinct species, there are several which appear to 

 agree in their crystalline forms. 



Now if we regard the Fontainbleau sandstone as a 

 variety of carbonate of lime, enveloping grains of 

 quartz ; and as we observe that the crystalline form 

 of the carbonate of lime is not altered by the presence 

 of this siliceous mixture, we may infer that the crys- 

 talline character of minerals is not affected by the 

 accidental presence of foreign matter in their com- 

 position, and consequently that minerals differing 

 3*2 



