In seams and tiny crevices this mineral, and often pyrolusite, grows in a branching manner, resembling tree-like or “mossy” masses. This is termed dendritic, and the growths of manganese minerals are called dendrites. One of the most curious of these is when the “mossy” growth is inclosed in chalcedony, making the so-called _moss agate_. These moss agates are abundant through the Rocky Mountains and are frequently cut for semi-precious stones. The finest ones however come from India and China. Manganite is found in the Lake Superior region, Colorado, etc. Rhodochrosite MnCO₃ Occurs in compact cleavable masses; hardness 4; specific gravity 3.5; color rose to dark red; streak white; luster vitreous; translucent on thin edges. This usually occurs in pink to red masses which cleave readily parallel to the faces of the rhombohedron. When it is found in crystals, which are rare, these too are rhombohedrons. It is usually found in veins as a gangue mineral with copper, silver or zinc ores. Its beautiful color and the fact that it effervesces in acid serve to distinguish this mineral. It is found at Branchville, Conn., at Franklin Furnace, N. J., and in veins with silver in Colorado, Nevada, and Montana. The Aluminum Group Though aluminum is one of the most abundant of all the metals, making some 8% of the crust of the earth, its union with other elements is so firm, that only recently have methods been found for getting the metal free. It was first isolated in 1846, but up to 1890 the extraction of aluminum was so expensive, that it could not be widely used. About that time electrical processes were applied to its extraction, and since then the price has steadily dropped, until now it is under $.20 per pound. It is very malleable, and ductile, and has high tensile strength. Exposed to the air, water or ordinary gases, it does not tarnish; and it is very light, an equal bulk weighing about a third as much as iron. The combination of lightness and strength, and the fact that it is a good conductor of electricity, have made it available for a wide range of uses, such as electrical apparatus, delicate instruments, boats, aeroplanes, and domestic utensils. It is an essential component of all the important rocks, except sandstone and limestone, and combines to a greater or less degree in a host of minerals. Though present in clays, shales, argillites, feldspars, and micas, it is only from bauxite that it has been successfully extracted. Aside from the small number of simple compounds of aluminum grouped here, it also takes a part in the make-up of a large series of minerals termed silicates, treated a little further on in this book.