“Inside the globe [there] exist mysterious forces, whose effects become apparent on the surface. Eruptions of vapors, glowing lava and new volcanic rocks…[]”
Alexander von Humboldt
At the end of the 19th century and after the victory of “Plutonism” in the great Granite War, geologists accepted the idea that igneous rocks originate from deep inside earth. However the great variability of volcanic and plutonic rocks, from dark basalt to light-colored granite, was difficult to explain, as Earth´s interior was assumed to be relatively uniform.
The “evolutionary tree” of igneous rocks (1954).
Whewell's Ghost 