1916] THE GROWTH OF ETCH-FIGURES 265 
the negative crystals by etching, and the experiments described in the 
preceding paragraphs appear to give some indications of the conditions 
under which this would take place in nature. In the laboratory we 
have a gentle corrosive—dilute hydrofluoric acid at 100°—acting for 
a considerable time upon a crystal, in nature we have a corrosive per- 
haps equally mild—water at a high temperature and pressure—acting 
upon a crystal for a much greater time, and the results are quite com- 
parable. It is true that the channels and negative crystals in the natural 
examples may be much larger than in the artificial, a result quite in 
keeping with the fact, which these experiments illustrate, that slow 
etching tends to make possible the production of larger etch-figures. 
Furthermore as a link in the chain of evidence, it is interesting to re- 
member that naturally etched crystals are occasionally met with, as, 
for example, that of spodumene from Alexander, N.C., described by 
Dana.” 
30 Zeitsch. fiir Kryst., 6: 509. 
