Form of Minerals Feuchtwanger: Treatise on Gems Page 35



	FORM OF MINERALS.	38

	poles, and the edges connected with them polar edges. The two other axes are named subordinate or lateral axes, and a plane passing through them is named the basis of the crystal. The two planes that pass through the principal and one of the lateral axes are named normal chief sections, and a plane through the chief axis intermediate to them, a diagonal chief section. The name tetragonal is derived from the form of the basis, which is usually quadratic. There are eight tetragonal forms, of which five are closed, —that is, bounded on all sides by planes, and of definite extent,—and three open, which in certain directions are not bounded, and consequently of indefinite extent. The description of the varieties is as follows, it being premised that a crystallographic pyramid is equivalent to two geometrical pyramids joined base to base.



	Closed forms.—(1.) Tetragonal pyramids (figs. 25, 26) are inclosed by eight isosceles triangles, with four middle edges all in one plane, and eight polar edges. There are three kinds of this form, distinguished by the position of the lateral axes. In the first these axes unite the opposite angles ; in the second they intersect the middle edges equally ;