PETROGRAPHY.

About twenty years ago petrography or lithology, the study of the structure of rocks, was added to our list of sciences. Harvard, almost alone among American colleges, has paid attention to this science. As if by magic, one versed in this wonderful science can look through solid rock and tell what lies hidden far within. The tool of the petrographist is a polarizing microscope, that is, an ordinary compound microscope in which two Nicol's prisms of Iceland spar are placed at a certain distance apart.

By placing a shaving of a rock, ground so as to be translucent, between the prisms, one can tell the composition of the rock, by examining the colors of the component parts of the shaving in polarized light.

By this new science we are enabled to ascertain the origin of a rock, the changes its component parts have undergone, and in short its complete history. It gives us information regarding decay in building stone, and points out the injurious matter therein. The minerals in a rock are shown, no matter how minute they may be. The history of meteorites, up to this time unknown, is revealed by this science.

Germany possesses the most famous scholars in petrography, among whom are Rosenbursch, Zirkel and Cohen. The foremost scholars in America are Drs. G. W. Hawes and M. E. Wadsworth, the latter being professor of petrography here at Harvard, which is the only American college employing a professor of petrography exclusively. The present chair is maintained by the generosity of Prof. J. D. Whitney, the geologist.

F. S. M.