Nothing is more remarkable than the great variety of geometrical shapes which the crystals of the same substance, derived from different localities or produced under different conditions, are observed to display. One of the commonest of minerals, calcite, carbonate of lime, shows this feature admirably; the beautiful large rhombohedra from Iceland, illustrated in Fig. 5, or the hexagonal prisms capped by low rhombohedra from the Bigrigg mine at Egremont in Cumberland, shown in Fig. 6, appear totally different from the “dog-tooth spar” so plentifully found all over the world, a specimen of which from the same mine is illustrated in Fig. 7. No mineral specimens could well appear more dissimilar than these represented on Plate III. in Figs. 6 and 7, when seen side by side in the mineral gallery of the British Museum (Natural History) at South Kensington. But all are composed of similar chemical molecules of calcium carbonate, CaCO_{3}; and when the three kinds of crystals are investigated they are found to be identical in their crystalline system, the trigonal, and indeed further as to the subdivision or class of that system, which has come to be called the calcite class from the importance of this mineral. [Illustration: _PLATE III._ FIG. 6.—Hexagonal Prisms of Calcite terminated by Rhombohedra. ] [Illustration: FIG. 7.—Scalenohedral Crystals of Calcite, “Dog-tooth Spar.” CRYSTALS OF CALCITE FROM THE SAME MINE, ILLUSTRATING DIVERSITY OF HABIT. (Photographed from Specimens in the Natural History Department of the British Museum, by kind permission.) ]