THE NAMING OF ROCKS 699 
many pages in a text-book are given to a discussion of earth- 
quakes, and only a small fraction of that space is given to the 
vastly more significant, slow earth movements which occur with- 
out earthquakes or with earthquakes as secondary phenomena. 
7. Finally, the foregoing plan has one further advantage. It 
emphasizes a great law of nature, the law of gradation-—the prin- 
ciple that ultimately there is no such thing as an absolute type. 
In this respect, petrography, if it will but use its opportunity, 
has an advantage over any other subject. In biology there have 
once been gradations between species, between families, between 
classes. Many of these gradations have been destroyed in the 
course of evolutionary processes. However, in petrography all 
of the gradations yet persist, and thus it furnishes the best illus- 
tration of the principle of gradation, a fundamental principle of 
nature. 
C. R. Van HIseE. 
Mapison, WIs., 
November 1899. 
