260 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 
Fic. 5—Stalagmo-Stalactite Shiloh Cave. ee ee chee Cele are 
(Mus. No. G. 884.) thombohedral. 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 change to a rhom- 
bohedral condition. This is net unlikely. But whatever the deter- 
mining causes, the case seems to furnish an instructive illustration of 

* Of. cit., p. 78. 

