Leonard James Spencer, 1911 Encyclopædia Britannica, Volume 28… (1911)
“ Zeolites are minerals of secondary origin and in most cases have resulted by the decomposition of the felspars of basic igneous rocks: in fact their chemical composition is somewhat analogous to that of the felspars with the addition of water. Nepheline and sodalite are often altered to zeolites. They usually occur as crystals lining the amygdaloidal and other cavities of basalt, melaphyre, &c. Usually two or more species are associated together, and often with agate, calcite and some other minerals. ”
