Then they are not volcanic.

If they were borne by icebergs, then they were once part of a glacier and some of them will probably show the flat surfaces with parallel scratches which are found on glacier-borne stones.

We examine them and find the characteristic scratched surfaces.

Then they have probably been brought to this place by icebergs.

But this does not exclude human agency, for they might have been brought by men to this place from some other where the icebergs had deposited them.