It should be borne in mind that only volcanic activity on an enormous scale would be plainly visible to us even with the powerful telescopes at our command. Ordinary eruptions such as occur on our own planet would be very difficult to detect. Since the escaping vapors would rapidly pass into the solid state and settle down upon the flanks of the crater-cones or vents, we would observe in general little if any change in an object unless we chanced to be looking at it at the time of the eruption, when it might appear to be temporarily obscured by a veil of vapors. What are the chances that we would be carefully observing at the precise time of an eruption, a minute marking, two or three miles in diameter, on a surface as large as all of North America, a surface that is covered with some 30,000 charted craters, numberless crater-pits, streaks, rays, spots, clefts and rills in intricate systems, mountain chains and valleys and a mass of intricate detail?