By 1850, A. Boisse, an early French geologist and meteoriticist, had put forth the basic _meteorite-planet_ hypothesis. According to this theory of his, meteorites are the fragments of a planet[12] that formerly orbited between Mars and Jupiter in what is now called the “asteroid belt.” And untold millions of years ago, this planet was shattered by some unknown but very great force, possibly collision with another celestial body. The structure of the meteorite-planet was considered to have been very much like that of the earth. The various divisions of recognized meteorites were believed to be representatives of the several concentric, or nested, shells of material originally making up the destroyed planet. These shells were progressively less dense with increasing distance from the center of the planet. Today Boisse’s theory is one of the most widely accepted as an explanation of at least one major category of the meteorites. Some modern investigators would insist that the meteorite-planet had a thin outer glassy shell from which the tektites came. Most of the larger fragments of the meteorite-planet, now called the _asteroids_, move so that the average asteroidal orbit very closely approximates the orbit of the original planet. But many of the smaller fragments follow paths in space that differ considerably from the original meteorite-planet’s orbit. Even some of the asteroids behave this way, either because of the high speeds they acquired at the time of disruption of the meteorite-planet, or because of the later influence of the major planets and particularly of the giant planet, Jupiter. [Illustration: A diagram (not drawn to scale) showing position of asteroid-belt with respect to the orbits of Mars and Jupiter. The asteroids with average orbits move within this belt. The non-typical asteroid indicated follows an orbit that brings it well inside that of the earth. There are a number of asteroids with such peculiar orbits. It is possible that in the past a nickel-iron asteroid in one of these orbits collided with the earth and produced the Canyon Diablo meteorite crater.] In fact, at the present time, several asteroids move well within the orbits of the earth and Venus. It is quite possible therefore that such a large meteorite crater as the one at Canyon Diablo, was produced by the prehistoric fall of one of these small members of our Solar System. If so, we have reason to believe that a core-fragment of the meteorite-planet came to earth at Canyon Diablo. For the extensive mining operations carried out there during the last half-century have shown that the projectile responsible for this greatest of all meteoritic shell-holes in the face of Mother Earth was a mass of solid nickel-iron, which in all likelihood was core material.