Into the Field Again: Re-Examining Charles Darwin's 1835 Geological Work on Isla Santiago (James Island) in the Galápagos Archipelago

In 1835 Charles Darwin's geological observations on Isla Santiago (James Island) in the Galápagos Islands led him to important insights as to the process by which different varieties of igneous rock might be produced from the same volcanic vent. His work figured in a tradition of interpretation that began with the work of George Poulett Scrope and would end in the twentieth century with the theory of magmatic differentiation of igneous rocks through the process of crystal fractionation. This article reports on the findings of an expedition to Isla Santiago in July 2007 during which we were able to locate samples of igneous rocks similar to those collected by Darwin. We have used these, together with Darwin's original specimens and transcriptions of his field notes, to examine how his understanding of the separation of the trachytic and basaltic series of magmas developed from his initial field observations through to publication of Volcanic Islands in 1844.

Ultrabasic Pillow Lavas from Cyprus

Small isolated masses of highly olivine-rich lavas within a dominantly basaltic series of pillowed flows have been found in the northern foothills of the Troodos Igneous Massif. The rock is characterized by abundant phyric olivine usually set in a glassy, basaltic groundmass. Both intrusive and extrusive varieties of the rock have been found. It is suggested that the rock originated as a melt, possibly from the Troodos ultrabasic complex. Comparison is made with the serpentine lavas of Turkey.

II.—The Red Zone in the Basaltic Series of the County of Antrim

One of the most beautiful features at the Giants' Causeway, from an artistic as well as a geological point of view, is the broad red zone that divides the Lower from the Upper Basalts. As is well known, this zone of lithomarge, bole, and laterite is remarkably persistent in north-eastern Ireland, and represents an interval of Eocene time when volcanic activity was lessened and when the basalts ceased to appear at the surface. At the same time, however, sporadic eruptions of rhyolite occurred, and some of the cones of acid lava supplied material for an interbasaltic conglomerate of rhyolite pebbles, which was discovered several years ago by Mr. A. McHenry near Glenarm.