The lava rock that's widely sold as a garden top dressing is nothing
like real
pumice. Pumice is porous, solidified rock foam, and quite water retentive,
whereas the lava rock isn't water retentive to any great degree.

This charming fact means that you can plan next summer's vacation
now: plan on
visiting Crater Lake in southern Oregon. The access road along Annie Creek to
the Rim Drive climbs to the caldera rim over enormous deposits of pumice, and
along US Highway 97 which runs up the east flank of the Cascades, NE
of Crater
Lake you go for miles through drift, banks, and hills of pumice thrown out by
Mt. Mazama when she lost her temper some 6000-7000 years ago.