This mineralogical study, published in 1853 by Darwin's German contemporary Wolfgang Sartorius von Waltershausen (1809-76) illustrates the author's dedication to interdisciplinary research and his desire for greater scientific rigour in geology. Seeking to understand the formation of palagonite, a mineral commonly found in rocks produced by submarine eruptions, Waltershausen realised he would also need to understand the precise composition of another class of minerals, the feldspars, on which Robert Bunsen, also at Goettingen, was working independently. Building on his earlier fieldwork, Waltershausen here reports detailed chemical analyses of minerals from olivine to titanium-iron oxides, dividing rocks into five types according to the proportion of oxygen they contain. He compares palagonites from Iceland and Sicily, and explains the role of erupted ash in producing volcanic edifices. His investigations allow him to propose a detailed model of metamorphic processes, taking into account such factors as pressure, temperature and presence of water.