4. A Theory of Volcanic Eruptions

From researches which have much engaged my attention for nearly twenty years, I am convinced that silica performs a very important part, not only in the formation of the earth's crust, but also in leading to violent subterranean movements. The low specific gravity of silicic acid, and of the rocks in which it predominates, would (if much of the internal earth were fluid) give rise to certain results, which I traced in an essay published in 1856, and also in a paper which was brought before the British Association for the Advancement of Science in 1861.