Note 13 — Chapter I · THEORY OF GEOLOGISTS.

It may be useful once for all to inform the reader that the term Rock is employed by Geologists in a technical sense. It is applied to every large mass of mineral matter that goes to form the Crust of the Earth, whether it be hard and strong, or soft and plastic. Thus, for example, gravel and clay, coal and slate, are called Rocks, just as well as limestone and granite. “Our older writers endeavored to avoid offering such violence to our language, by speaking of the component materials of the Earth as consisting of rocks and soils. But there is often so insensible a passage from a soft and incoherent state to that of stone, that Geologists of all countries have found it indispensable to have one technical term to include both, and in this sense we find roche applied in French, rocca in Italian, and felsart in German. The beginner, however, must constantly bear in mind, that the term rock by no means implies that a mineral mass is in an indurated or stony condition.”—Lyell’s Elements of Geology, p. 4.