On an Arenaceo-calcareous Substance. 81 



sidered as the elements of all these capricious appearances ; the 

 one consisting of a conical concretion, and the other of a lenticular 

 one, analogous to the stalactite and stalagmite of mineralogists. 

 These, combined and modified in different ways, produce the 

 several varieties of form that are found in this place. 



These concretions are formed of carbonate of lime, containing 

 sand united by that cement, in the same manner as it occurs in the 

 Fontainebleau spar. It appears difficult at first sight to account 

 for the stalactitic shape, since these concretions are neither formed 

 in cavities nor in a perpendicular position. They lie, on the con- 

 trary, in a direction but little inclined to the horizon, and are ge- 

 nerated in the rnidst of the sandy stratum. It seems equally 

 difficult to account for the presence of the carbonate of lime; but 

 it is natural to suppose that water saturated with that substance 

 finds its way through fissures or intervals in different parts of the 

 bank ; although it is not easy to conjecture whence it originates. 

 It would naturally be imagined that the calcareous solution thus 

 trickling through the sand, should diffuse and lose itself so as to 

 form with it a loose admixture ; or else thus consolidate the parts 

 within its reach into a calcareous sandstone. But it is not un- 

 likely that this partial formation is determined, in some measure, 

 by the innumerable surfaces of the sand, already perhaps contain- 

 ing calcareous particles, and offering bases on which the earth 

 in the solution is quickly deposited by a species of crystallization, 

 thus checking that diffusion. The rudiment of a stalactite, once 

 formed, serves perhaps as a conductor to the fluid, which is thus, 

 by a continuation of the same process, enabled to prolong these 

 concretions, in some cases, even to the length of three feet. This 

 explanation, however, does not apply to the lenticular stalagmite 

 and its modifications ; nor am I at present able to explain how 

 this appearance, which, in ordinary cases, results from the diffu- 

 sion of successive drops falling on an exposed surface, should here 

 occur in a close mass of sand. I must add that there is no ap- 

 parent difference in the position of the two modifications ; both 

 being found confusedly together, and without that mutual relation 

 which occurs in the common concretions of this nature : and I 

 Vol.. XVI. G 



