A geologist describes the structure of volcanic heapings for interpretation of history. A geophysicist,describing structural development, has in view the basis of experience for forecasting evolution of construction that is to come, in a living volcanic edifice.A volcanologist in Hawaii observes changes in the structure of a volcano by direct experience. He watches the addition of gushes of gas and of extruded magma from within the so‐called crust of the Eart