26o 



Field Columbian Museum — Geology, Vol. i. 



Molecular Arrangement of Stalactites and Stalagmites.— The 

 substance composing the stalactites and stalagmites of this cave is 

 generally made up structurally of fibres radiating outward from the 



center. The fibres pass unin- 

 terruptedly through the concen- 

 tric rings of growth and the 

 structure is doubtless, therefore, 

 as pointed out by Merrill,* of 

 secondary origin. The fibrous 

 substance is not, however, 

 aragonite, but calcite. In con- 

 trast to the forms possessing 

 this structure are many whose 

 substance has a wholly coarsely- 

 crystalline structure exhibiting 

 an all-pervading rhombohedral 

 cleavage. Intermediate stages 

 between these two extremes can 

 be seen in many cases. Of 

 especial interest are stalagmites 

 exhibiting a structure like that 

 shown in Fig. 6. This figure 

 shows a cross section of a 

 stalagmite, the peripheral por- 

 tions of which are fibrous in 

 structure while the central are 



Fig. 5— Stalagmo-Stalactite, Shiloh Cave. , , . , , T r , 



(Mus. No. G. 884.) rhombohedral. I am of the 



opinion, though I know of no way either of proving or disproving 

 it, that such a structure is evidence of a progressive change in the 

 molecular arrangement of the substance toward a more stable con- 

 dition. It is certain that it is in the older portion of the stalagmite 

 that the molecules, are arranged along the rhombohedral planes, and 

 I have never found the positions reversed. That the rhombohedral 

 condition is more stable than the fibrous seems to be indicated by 

 the fact that the former is characteristic of the oldest and most meta- 

 morphosed calcite-bearing rocks. Prof. D. G. Elliot has suggested 

 to me that pressure on the internal substance of the stalagmite may 

 also be largely instrumental in bringing about the ; $feange to a rhom- 

 bohedral condition. This is not unlikely. But whatever the deter- 

 mining causes, the case seems to furnish an instructive illustration of 



*Op. cit.,x>. 78. 



