teor displays
of Earth. (And there is still a standing argument among the men of the
Belt as to whether that sort of action can be called "weathering".) Most
of the collisions tend to cause fracturing of the surface, which results
in jagged edges. A man in a vacuum suit does not push himself against a
surface like that with any great velocity.

* * * * *

St. Simon knew to a nicety that he could propel himself against a bed of
nails and broken glass at just the right velocity to be able to stop
himself without so much as scratching his glove. And he could see that
there was no ragged stuff on the spot he had selected. The slanting rays
of the sun would have made them stand out in relief.

Now he was clinging to the surface of the mountain of rock like a bug on
the side of a cliff. On a nickel-iron asteroid, he could have walked
around on the surface, using the magnetic soles of his vacuum suit. But
silicate rock is notably lacking in response to that attractive force.
No soul, maybe.

But directly and indirectly, that lack of response to magnetic forces
was the reason for St. Simon's crawling around on the surface of that
asteroid. Directly, because there was no other way he could move about
on a nonmetallic asteroid. Indirectly, because there was no way the big
space tugs could get a grip on such an asteroid, either.

The nickel-iron brutes were a dead cinch to haul off to the smelters.
All a space tug had to do was latch on to one of them with a magnetic
grapple and start hauling. There was no such simple answer for the
silicate rocks.

The nickel-iron asteroids were necessary. They supplied the building
material and the major export of the Belt cities. They averaged around
eighty to ninety per cent iron, anywhere from five to twenty per cent
nickel, and perhaps half a per cent cobalt, with smatterings of
phosphorous, sulfur, carbon, copper, and chromium. Necessary--but not
sufficient.

The silicate rocks ran only about twenty-five per cent iron--in the form
of nonmagnetic compounds. They averaged eighteen per cent silicon,
fourteen per cent magnesium, between one and one point five per cent
each of aluminum, nickel, and calcium, and good-sized dollops of sodium,
chromium, phosphorous, manganese, cobalt, potassium, and titanium.

But more important than these, as far as the immediate needs of the Belt
cities were concerned, was a big, whopping thirty-six per cent oxygen.
In the