336 



Prof. Rogers added a brief sketch of facts which he had 

 observed in the growth of stalactites. 



A drop of water, charged with carbonate of lime, is seen to 

 form at a particular point of the roof, and after its descent, another 

 drop, by the same mechanical causes, takes its place. It is not 

 necessary to suppose a hole around which the concretion may 

 collect. Usually there is none. At the margin of the drop 

 where it thins away to a film, evaporation and the loss of car- 

 bonic acid combine to cause a precipitation of part of the dissolved 

 carbonate, which, on separating, attaches itself to the rock in the 

 form of a very delicate white ring, corresponding to the margin 

 of the liquid. Each succeeding drop deposits a similar ring in 

 contact with and beneath that already formed, until the whole is 

 prolonged downwards in the shape of a quill-like tube. This, 

 from its vertical position, invites the water of the adjoining part 

 of the roof to descend along its outer surface, and now an exte- 

 rior and more rapid growth begins. Usually, the former process 

 continues to operate for a long time after the external growth 

 has commenced ; so that the stalactite, in some cases, retains 

 its open central canal until it has reached a length of a foot or 

 more, and a diameter at its base of two or three inches. As the 

 water, which flows along the outside of the tube, parts at each 

 step with a portion of its calcareous charge, and thus grows con- 

 tinually less capable of forming the deposit, the rate of deposi- 

 tion must diminish somewhat regularly from the upper to the 

 lower end of the mass. Hence it is that stalactites, formed in 

 positions where their growth on all sides is freely permitted, 

 have always a sharply conical or tapering form. 



The drops which fall from these pendants to the floor, still 

 retain a portion of carbonate of lime in solution ; but as the shock 

 of the impact and the spreading of the liquid greatly favor the 

 escape of its carbonic acid, a further deposit must be formed in 

 this position, and thus the stalagmite grows upwards to meet the 

 stalactite growing downwards, until in many cases they unite 

 to form a column reaching from the floor to the ceiling of the 

 cave. 



As in general the infiltering water follows the joints and planes 



