W. A. Tarr—Cone-in-Cone, 2G9 



important source of pressure. The change of aragonite 

 to calcite would give an adequate source of pressure and 

 seems to be the most likely source. What sources of 

 pressure other than this exist in the rocks associated with 

 the cone-in-cone are not evident to the writer. Those 

 which are there certainly function, as it is doubtful if 

 much movement could have taken place without some 

 pressure. 



Origin of the Conical Form. 



The origin of the conical shape of the cone-in-cone is 

 extremely puzzling. If the angle of the slope of the cones 

 was always the same, and if it was similar to the cleavage 

 angle of the material composing it, the problem would be 

 much simpler. But the cleavage angle in calcite is 

 approximately 105°, which is far greater than that of 

 the cone-in-cone. This may mean that the cone-in-cone is 

 not related to the cleavage, or, that the original substance 

 was not calcite and thus had a different cleavage angle. 

 As the original substance was calcium carbonate, it may 

 be that aragonite was the original mineral. But the best 

 cleavage in aragonite is pinacoidal whereas the cleavage 

 parallel to the brachydome is poor. Since a series of 

 brachydomes is possible on aragonite, forms with acute 

 domes might have a poor cleavage with angles similar to 

 those of the cone-in-cone. The poor cleavage of aragonite 

 along this plane, however, does not favor the possibility 

 that the initial form of the cone-in-cone was due to it. 



Another possibility, if the original form of the material 

 was aragonite, should be considered. Radiating struc- 

 tures are very common in aragonite. This structure may 

 give rise to stalactites, spherical masses, or radiating 

 tufts. The tufts have considerable significance in this 

 problem. If the mineral were deposited along a plane 

 with deposition occurring at many points, a layer con- 

 sisting of an aggregate of such radiating tufts would 

 result. The spacing of the tufts along the plane would 

 determine their size, and incidentally the angle of the 

 outermost fibers on the tuft. Close spacing would give a 

 sharp angle, and wide spacing a low angle. If, now, 

 pressure were brought to bear on the layer, movement 

 would be greatly aided by the fact that the outermost 

 layers would have the distinct cleavage of the pinacoidal 

 and prismatic faces. The fact that the cone surface is 



