Sometimes they are filled, or partially filled, with mineral matter introduced at the time of eruption, or subsequently brought in a state of solution and deposited there by water Filtering through the rock: this forms what is called amygdaloidal lava.
"Geology"
James Geikie
"Is it not possible" he asks, "that the enjoyment of art for its own sake, the calm and full satisfaction we are plunged into by the contemplation of a beautiful statue or of a perfect monument, things that do not belong to us and that we shall never see again, which excite no sensual desire, which can profit us nothing-is it not possible that this satisfaction may be the pale gleam of a different consciousness Filtering through a fissure of that consciousness of ours which is built up of memories?"
"Life and Writings of Maurice Maeterlinck"
Jethro Bithell
The answer perhaps is Filtering now in the next room, or it may be a question of several weeks' experiment.
"I Walked in Arden"
Jack Crawford