Lava rock varies widely in chemical composition and physical qualities.
In some instances lavas are so light and frothy or contain so large a
proportion of easily oxidizable material that they are wholly unsuited
for concrete work. In general, the lava rock found in the Northwestern
States is a suitable substitute for gravel. Rhyolite, a light colored
volcanic rock, and many of the darker colored basaltic lavas can well be
used for concrete for building purposes.