The later history of expanded spherulites (thunder eggs), as disclosed by the minerals found within their central cavities, is interpreted as due to three distinct generations of crystallization. The first, consisting of a lining of chalcedony, was based on residual silica within the cavity after the crystallization of the feldspar-cristobalite cover. The second, in the shape of chalcedonic plates and associated drusy quartz, was due to the migration of silica from the immediately adjacent lava, aided by the diminution of the internal pressure which had given rise to the expansion of the spherulite. The third followed the introduction from purely external and, possibly distant sources, of solutions bearing quartz (in large quantities), calcite, haematite and (rarely) laumontite.
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