The amphiboles are a group of minerals made up of the same chemical elements as the pyroxenes, but with the molecular arrangement different, which appears in the forms of the crystals. The commoner ones are all monoclinic but contrast with the pyroxenes as follows. Amphiboles are long and slender crystals, while pyroxenes are short and stout; amphiboles are six-sided, while pyroxenes are eight-sided; amphiboles have the two perfect cleavages intersecting at 55° and 125°, while those of pyroxene intersect at 87° and 93°. With the above in mind it is easy to place the minerals in their proper group, but inside the group it is not always so easy to distinguish one from another. This group is associated rather with metamorphic rocks than with igneous rocks, with which the pyroxenes are mostly associated. The three commoner minerals of the group are tremolite, actinolite, and hornblende. Tremolite (CaMg)₃(SiO₃)₄ Pl. 37 Occurs in long prismatic crystals or in columnar or fibrous masses; hardness 5.5; specific gravity, 3; color white to gray; luster vitreous; transparent on thin edges. The long prismatic crystals of tremolite occur especially where dolomitic limestones have been altered by metamorphism. Sometimes these crystals grow side by side, making fibrous masses, where the long slender crystals can be picked apart with the fingers, and yet are flexible, and tough enough so that they can be felted together. This is termed asbestos, which, because it is infusible and a poor conductor of heat, is much used to make insulators, fire-proof shingles, and all sorts of fireproof materials. The varieties in which the crystals are finer and silky in appearance, like the one illustrated on Plate 38 are termed _amianthus_. There are other minerals, such as actinolite and serpentine, which occur in the same manner, and are also called asbestos, the serpentine variety being just now the most important commercially. Tremolite is found at Lee, Mass., Canaan, Conn., Byram, N. J., in Georgia, etc. Actinolite (CaMgFe)₃(SiO₃)₄ Occurs in radiating crystals, or in fibrous masses; hardness, 5.5; specific gravity 3; color pale- to dark-green; luster vitreous; translucent on thin edges. Except for its green color, this mineral is very like tremolite. The difference between the two is due to the small amount of iron in the actinolite. It is usually found in schists, and the radiating character of the crystal groups is enough to determine the mineral, if it is already clear that it is one of the amphiboles. Occasionally it occurs with the crystals parallel to each other, making one of the forms of asbestos. Actinolite is found at Warwick, Edenville, and Amity in Orange Co., N. Y., at Franklin and Newton, N. J., Mineral Hill and Unionville, Penn., Bare Hills, Md., Willis Mt., Va., etc. Hornblende (CaMgFe)₃(SiO₃)₄CaMgAl₂(SiO₄)₃ Pl. 37