The White Crystals by Howard Garis
Being an Account of the Adventures of Two Boys
This is an early Mystery/Adventure published in 1904 by the author of most of the original Tom Swift books and the Uncle Wiggly series of books. It was relatively rare. It just finished up and went into Project Gutenberg. The book involved salt mining in upper New York.
http://www.gutenberg.org/ebooks/38558
http://en.wikipedia.org/wiki/Howard_R._Garis
Greg Weeks