Foliation

Foliation, a term restricted by Darwin, and subsequently by geologists, to the alternating and more or less parallel layers or folia of different mineralogical nature, of which the crystalline schists are composed. It differs from Cleavage (q.v.), which is applied to certain superinduced divisional planes that render a rock fissile; and from lamination, in which the planes of separation in a rock are the result of deposition in successive layers. The folia of a schistose rock may be composed of only one mineral, but most commonly they consist of two or more; they are conspicuously lenticular, thickening and thinning out, and reappearing after an interval on the same or a different plane. These alternately lenticular folia are usually more or less closely welded or felted into each other, so that they are not readily separable; and they frequently present the appearance of being puckered or crumpled. The crystalline texture and the foliated character of the schists distinguish them at once from any ordinary bedded 'fragmental rock.'