82 On an Arenaceo-ca/careoits Substance 



may also add that the purer forms are as accurate as those found 

 in caverns : the stalactite being a perfect prolonged cone, and the 

 stalagmite a thin and round flat cake. 



The fractured surface of the specimens varies ; evidently in con- 

 sequence of the greater or less quantity of sand entering into the 

 composition, and of variations in the fineness of its particles. It 

 is rarely so distinctly laminar as even those specimens of Fon- 

 tainebleau spar which are most charged with sand; and, in general, 

 it is so much like an ordinary sandstone that the presence of the 

 calcareous ingredient would not be suspected. As such indeed it 

 was originally sent to me, being supposed a corroded specimen of 

 sandstone, of which an internal structure was detected by the usual 

 causes of wasting. 



It is perhaps of little use to state the relative proportions of 

 the ingredients, which are moreover subject to variations ; but the 

 average of the specimens I examined gave CO parts of sand in 100 

 of the stalactite. 



Considering the crystalline arrangement of the carbonate of lime, 

 as indicated by the platy fracture of the specimens, and the ana- 

 logous circumstances under which the mineral of Fontainebleau is 

 found, it might have been expected that geometric forms should 

 also be found with the rest in the sand banks of this spot. None 

 such have, however, yet been discovered ; and the resemblance re- 

 mains confined to the composition and internal structure. The 

 present appearance can therefore only be considered as an analogy; 

 an instance of the possibility, as that is of the actual existence, 

 of a compounded crystal, in which the presence of a foreign body 

 does not impede the crystalline tendency of the crystallizable in- 

 gredient, although the latter is so much inferior in quantity to the 

 intruding material. It is probable that with a general resemblance 

 in both cases, namely, the presence of a calcareous solution in a 

 mass of sand, there is an essential difference, in one particular, 

 between the circumstances under which the crystal of Fontainebleau 

 and the stalactite of Delvine are formed. From general principles, 

 Ave should conclude, that, in the former, the mixture of the sand 

 and the solution was preserved in a constant semifluid state, fresh 



