It comprehends any rock in which round or almond-shaped nodules of some mineral, such as agate, chalcedony, calcareous spar, or zeolite, are scattered through a base of wacke, basalt, greenstone, or other kind of trap.
"The Student's Elements of Geology"
Sir Charles Lyell
Minerals of the zeolite family, the composition of which has already been alluded to in Chapter 28, occur in amygdaloids and other trap-rocks in great abundance, and Daubree's observations have proved that they are not always simple deposits of substances held in solution by the percolating waters, being occasionally products of the chemical action of that water on the rock through which they are filtered, and portions of which are decomposed.
"The Student's Elements of Geology"
Sir Charles Lyell