The rhombohedron fig. 1, which may thus be seen to belong to the second order, is readily recognised by having its faces g making the same angle with the vertical axis A B (fig. 2) as the terminal edges of the ground-form, so that in a combination the terminal edge of the ground-form is truncated by the plane g. This rhombohedron, which is called the first obtuser, has the angles at the terminal edges 135° 57', and has with the same breadth its vertical axis one-half that of the ground-form. It is one of the most common of the rhombohedrons, and is frequently found alone, but still more frequently in combination with the hexagonal prism, producing the form seen in fig 7. It occurs frequently at Andreasberg, in the Harz, and in the mines of Derbyshire.
The rhomboliedron fig. 3 is also of the second order, and is called the first obtuser : its terminal edges correspond with the long diago nals of the faces of the ground-form, and therefore, with the same breadth, its vertical axis is double that of the other : the inclination of the faces at the terminal edges is 78° 51'. Iu combination with the ground-form, if the faces f predominate the form P appears as trun cations of the terminal edges ; if P predominates, the faces of f produce truncations of the six lateral angles, the edges of intersection being parallel to the inclined. diagonals of P for two faces, and with the horizontal one for the third.
The rhombohedron fig. 4 bears to fig. 3 the same relation as this does to the ground-form, the terminal edges of the first corresponding with the inclined diagonal of the second : the inclination of the planes to each other in the terminal edges is 65° 50'.
Fig. 5 is one of the most common scalinohedrons, and is commonly known as the Dog's-Tooth Calespar, and is found frequently in Derby shire and other localities. It bears a close connection with the
rhombohedrons P and in, having the lateral edges of the first and terminal edges of the latter, so that in combination with the first the form fig. 8 is produced, and with the second it forms a bevelment of the terminal edges : the inclinations of the faces in the terminal edges are respectively 104° 38' and 144° 24'. This form frequently occurs as twins, formed by two crystals growing on each other, their principal or vertical axes being in the same right line, and the two crystals so situated that the obtuser terminal edges of the one abut on those of the other, and the acuter on the acuter.
This mineral may be recognised by its perfect cleavage parallel to the faces P : the specific gravity of the purest crystals is ; and the hardness is in the scale of 31oha 3, being situated between gyps and fluor-spar. It is of itself colourless, but frequently occurs of various tints of yellow, green, red, brown, and even black, from the admixture of impurities. Its glance is vitreous, with the exception of the terminal face o, which generally presents a mother-of-pearl lustre. It is usually more or less translucent, and when transparent produces in a remarkable degree the double refraction of light: this property is best seen in the varieties obtained from Iceland, and hence known as Iceland-Spar, and occurs as the ground-form, being in fact merely broken fragments of other larger crystals.
The following are the names of the more common varieties of Cal careous Spar :—Iceland Spar, Satin Spar, Chalk, Rock Milk, Calcareous Tufa, Stalactites, Stalagmite, Limestone, Oolite, Pisolite, Argentine, Fontainbleau Limestone, White and Clouded Marbles, Statuary Marble, Compact Limestone, Stinkatone, Anthmconite, Plumbo-Calcite, Mine ral Agarie, &c.