a chemistry question about salt prints

Purely out of curiosity, does anyone know what form of silver makes the image of a salt print? Is it elemental silver particles that appear different shades of brown due to their size, or is it a silver compound? If a silver compound, what might it be? Also, the color seems to be affected by the specific paper that I have used. What causes this?

Thanks

--
Bill Leigh
wrleigh@att.net