134 



spar by the energetic segregation of the crystalhzing carbonate of 

 lime; the sand being inert matter which was forced by the calca- 

 reous salt to enter into the crystalline form of the spar. 



Simihxr illustrations were adduced from chemical experiments 

 and observations in which spheroidal forms result, and also in 

 which foreign bodies are forced to enter into the structure of 

 crystals. He quoted the experiments of Beudant, which he had 

 repeated, in which Prussian blue was included in crystals of 

 nitre and alum. He also alluded to the effects of different men- 

 strua, in modifying the forms of crystals or totally changing their 

 forms, instancing the crystallization of sea salt in the foi'ms of the 

 regular octahedron in a solution of urea, whereas the cube is its 

 usual form. 



Dr. A. A. Hayes followed Dr. Jackson, in remarking on the 

 concretions called claystones. Having incpiired of Dr. J. how 

 large was the proportion of sand in the Fontainbleau crystallized 

 sandstone, and received for answer about fifty per cent., he said 

 that he had often examined the spots where they were forming, 

 and had noticed a growth equal to the size of a garden bean, to 

 take place in the course of two or three weeks of wet, spring- 

 time weather. To form a just conception of the conditions, the 

 fact must be kept in view, that the beds containing them are 

 composed of fine silts, and in the case immediately under view, 

 these were arranged in planes of deposition of alternate courses, 

 covered by much finer material, in layers of different thickness ; 

 so that the mass was stratified ; the coarser layers being very 

 permeable to water. The rounded forms, often strongly resem- 

 bling organic remains, are found resting between these layers, and 

 a condition necessary to their formation is, the presence in the 

 layer or rock above them of abundance of carbonate of lime. 



The force exerted by some salts in their tendency to crystallize 

 is brought into view only when we study their formation, and 

 carbonate of lime is one of the constantly occurring salts which 

 well illustrates, in a remarkable manner, this power of assuming 

 regular forms. As has been stated, with fifty per cent, of its weight 

 of sand, it forms regular rhomboids, but the more recent obser- 

 vations of some African travellers, who found their progress im- 

 peded by " stone plants," six or eight inches high, formed of 



