1894.] 



NATURAL, SCIENCES OF PHILADELPHIA. 



109 



at my command (about 8 milligrams) was found to be 2.285. It is 

 soluble in water and gives the reactions indicating sulphur trioxide 

 and calcium oxide. The incrusting small crystals are, therefore, 

 selenite. 



Fig. 2. Crystal and twin forms of selenite, x 45. 



I am indebted to Professors Sharp and Libbey for the material 

 studied and also for the photographs of the volcanic cave. 



Plate VI shows the entrance to the cave with its overhang- 

 ing roof of porous basalt from which are suspended the irregularly 

 gnarled rods of volcanic stalactite; on the floor are scattered 

 fantastic-shaped volcanic stalagmites which seem to be much 

 thicker than the pendant rods above. 



