To the student of rocks the forms and relations of crystals and of crystal aggregates precipitated from a cooling magma are in large measure the expression of the physical conditions under which the magma solidified. This was thoroughly appreciated by the pioneers in petrology, who observed that as the physical conditions of freezing of such magmas varied, so also did the resulting products of crystallization, with respect both to the kinds of crystals formed (mineral composition) and to the habits and relations between the crystals (rock texture). Of the different kinds of crystals thus studied none has received more attention than the spherulites; and yet our knowledge of them is still incomplete, especially of the hollow spherulites or lithophysæ, the best examples of which have been found in the obsidian of Yellowstone National Park. These were studied many years ago by Iddings2 in detail and with special reference . . .