On an /Irenaceo-calcarcous Substance. 83 



calcareous matter succeeding as the first was deposited, so as to 

 permit a slow arrangement of the carbonate of lime ; while, in the 

 latter, we are certain that the precipitation of this salt is hurried 

 by the rapid flow and absorption of the percolating fluid. In the 

 ordinary stalactite of caverns, the same effect results from the 

 rapid evaporation of the water, and from the mechanical descent 

 of that part of the solution which has not had time to deposit its 

 contents in its passage downward. This opinion is supported by 

 the situation in which the Fontainebleau crystal is found, namely, 

 the fissures of a sandstone ; in which it is easy to conceive that 

 state of things which I have here suggested. The occasional 

 variations that occur in this substance still farther confirm this 

 view. Mineralogists will immediately perceive that I allude to 

 that case in which the crystal consists partly of an arenaceous and 

 partly of a pure carbonate of lime. Here, it is probable, that the 

 fluid has so far prevailed in the fissure as to overtop the sandy 

 mixture, thus admitting the continuation of the arrangement from 

 the mixed to the pure part of the solution. It is probable that 

 there would not be much difficulty in putting this suggestion to 

 the test of experiment ; by filling with sand those pools in the 

 Spar Cave of Sky, in which, as I have shewn in another place, 

 (Western Islands,) the formation of calcareous spar takes place. 



The appearances now described will serve to illustrate another 

 circumstance occurring, not very unfrequently, in sand, in different 

 parts of England. The substances in question are found, among 

 other places, in the sand that lies above the fossil bones of Norfolk ; 

 and they have been ranked, improperly, with organic remains. — 

 They consist of long cylinders, or tubes, of different dimensions, 

 sometimes formed of one crust or layer, at others of more; in 

 which latter cases partial cavities sometimes occur between the 

 layers. On analysis, they will be found, like the stalactites of 

 Delvine, to be composed of sand agglutinated by carbonate of 

 'ime, or rather they must be considered as calcareous stalactites 

 entangling sand. The calcareous ingredient is often, however, 

 distinctly visible in these; forming a lamina among the successive 

 coats, in which the crystalline particles are seen radiating from 

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