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HANDY-BOOK OF ROCK NAMES.
THE term Rock, in a geological sense, includes I every solid substance that is an ingredient, or forms part, of the earth. Thus loose sand, clay, peat, and even vegetable mould, geologically speaking, are rocks. Jukes thus defines a rock :-—"A mass of mineral matter consisting of many individual particles, either of one species of mineral, or of two or more species of minerals, or of fragments of such particles. These particles need not at all resemble each other in size, form, or composition; while, neither in its minute particles, nor in the external shape of the mass, need a rock have any regular symmetry of form.” Rocks are most variable in condition and structure; soft or hard, loose or compact, friable or tenacious, coarse or fine, crystalline or homogeneous; or they may be scoriaceous, vesicular, hyaline, &c. &c.
Rocks may be chemically, mechanically, or organically formed, or two or more of these combined; they may be stratified or unstratified, igneous or aqueous, or partaking of the nature of both. Various classifications have been adopted by different writers on the subject, each taking dif-
BRIEF DESCRIPTIONS OF THE ROCKS.
ENGINEER DIPLOMATIST, TRIN, COLL. DUBLIN; MEMBER OF THE ROYAL IRISH