Common Minerals and Rocks — Themes and Context

A Teacher's Framework for Geology

Crosby's introduction explicitly addresses the science teacher, not the casual reader. He organizes geology into three divisions—dynamical, structural, and historical—and states that his course will cover only the first two. This pedagogical choice reflects the book's origin as Number XII in the Boston Society of Natural History's 'Guides for Science-Teaching' series. The author repeatedly uses analogies to biology, comparing dynamical geology to physiology and structural geology to anatomy, to make unfamiliar concepts accessible. He also emphasizes that studying present-day causes (like rivers and volcanoes) allows geologists to interpret past effects, a principle he calls 'the light of the experience thus gained.' This cause-and-effect reasoning is the backbone of his teaching method.

Systematic Classification of Silicate Rocks

The excerpts include a detailed table summarizing the mineralogical composition of fourteen silicate rock species, from gneiss to greensand. Each species is listed with its constituent minerals—for example, gneiss may contain orthoclase and quartz, or orthoclase, quartz, and mica. Crosby also notes physical traits: hydrous silicate rocks (talc schist, chlorite schist, greensand, serpentine) are 'distinctly lighter' than anhydrous ones with the same silica proportion, and are characterized by 'softness, smooth feel, and green color.' He describes serpentine as fine-grained, massive, compact, and tough, with colors from dark green to light greenish-yellow. These observations tie directly to the book's stated goal of teaching identification of common rocks.

Eruptive Rocks and the Role of Melting

Crosby defines eruptive or unstratified rocks as those formed by cooling of melted material from deep within the earth. He distinguishes between lava that flows out on the surface and dikes that solidify in fissures. This section also reveals his attention to historical debates: he notes that geologists once thought all serpentine was eruptive, but now many consider it sedimentary in some cases. He adds that serpentine can form by alteration of olivine basalt. Such remarks show Crosby incorporating contemporary scientific discussion, not just static facts. The excerpts do not include the full treatment of eruptive rocks, but the pattern of linking composition, texture, and origin is consistent throughout.

Readers approaching Common Minerals and Rocks should note that Crosby's systematic tables and classification schemes are central to his teaching method. The book is best used as a reference alongside physical specimens, as the descriptions emphasize observable traits like color, hardness, and reaction to acid. The author's habit of linking each rock's properties to its formation process rewards careful reading of the introductory chapters, which establish the cause-and-effect framework that governs the entire guide.