COMPOSITION OF ROCKS. li 
These analyses may be used for a variety of purposes. For ex- 
ample, they can help in tracing the change from an average igneous 
rock to an average sediment. They suggest something as to the 
characteristic features which distinguish a good building stone from 
other limestones and sandstones. They are applicable to the dis- 
cussion of a variety of large theoretical problems like that chosen by 
Professor Joly. These considerations alone justify their publication 
here. 
