It is always found in seams and cavities in lavas, as at Bergen Hill and Weehawken, N. J., Westfield, Mass., in the Lake Superior region, etc. Natrolite Na₂Al₂Si₃O₁₀ + 2H₂O Plate 44 Occurs as bristling crystals in seams and cavities in lavas; hardness, 5.5; specific gravity, 2.2; colorless; luster vitreous; transparent on thin edges. Natrolite occurs as beautiful bristling tufts of needle-like crystals, each crystal an orthorhombic prism with a very low pyramid on the end. This mineral is so easily fusible that it can be melted in a candle flame, giving to the flame the characteristic yellow color due to sodium. In hydrochloric acid it dissolves to a gelatinous mass. It is always a secondary mineral in cavities and seams in disintegrating lavas, and the tuft-like manner of growth is so characteristic, that once seen, it will always be recognized. Natrolite is found at Weehawken and Bergen Hill, N. J., at Westfield, Mass., in the Lake Superior region, etc. Stilbite H₄(CaNa₂)Al₂(SiO₃)₆ + 4H₂O Pl. 44 Usually occurs in sheaf-like bundles of fibrous crystals; hardness, 5.5; specific gravity 2.2; colorless to white, yellow or brown; luster vitreous; transparent on thin edges. Stilbite crystals are really monoclinic, but on account of almost universal twinning, appear as if orthorhombic. Like the two foregoing minerals, stilbite is found in the seams and cavities of disintegrating lavas. It is readily recognized by its habit of forming in sheaf-like bundles of fibrous crystals. It may also, but more rarely, occur in radiating masses. In hydrochloric acid it is completely dissolved. It is found in lavas, at Weehawken and Bergen Hill, N. J., in the Lake Superior region, etc. Calcium Calcium is one of the most abundant of metals, but never occurs as such in nature, nor is it used as a metal by man. In its metallic form it is yellowish-white, and intermediate between lead and gold in hardness. Exposed to air it soon tarnishes by oxidation, and in water rapidly decomposes the water, forming the oxide. However, it has a great affinity for other elements, and makes a large number of minerals, the most important of which are calcite, aragonite, gypsum and fluorite, while it is an essential component of some garnets, anorthite, epidote, amphibole and pyroxene. It is very widely distributed as limestone, and is found in solution in most all natural waters, and in the shells and bones of many animals and some plants. Calcite CaCO₃ Pl. 45 Occurs in well defined crystals in incrustations, and in stalactitic, oolitic, and granular masses; hardness, 3; specific gravity 2.7; colorless to white, or when impure, yellow, brown, green, red or blue; luster vitreous to dull; transparent on thin edges.