470 



MINERALOGY. 



Moh' 

 crystallo- 

 graphy. 



Oryctogno- and the systems of crystallization* derived from these, 

 are denominated rhomboidal, pyramidal, prismatic, and 

 tcssular. Connected with the rhomboids there are se- 

 ries of scalene iix-sided pyramids ; and the series both 

 of rhomboids and of the scalene pyramids are termina- 

 ted by regular fix-sided prisms, which are distinguish- 

 ed from each other by their positions. The pyramids 

 with square bases form series ; and their limiting fornii 

 or last member of the series, is the rectangular four- 

 sided prism. Each pyramid with a square base has 

 depending on it three scalene eight-sided pyramids ; and 

 the limiting forms of these scalene eight-sided pyra- 

 mids, are tineqitiangitlar cigfil-sided prisms. The oblique 

 four-sided prism has depending on it numerous oblique 

 based pyramids, and oblique prisms. The hexahedron 

 has depending on it the regular octahedron, including the 

 tetrahedron, the rhomboidal dodecahedron, and the icosite- 

 trahedron. The forms that arise from the hexahedron pro- 

 duce among themselves various combinations ; but they 

 admit into them no form, tvhich is either a rhomboid, 

 or a four-tilled pyramid tvilh a square base, or an oblique 

 Jour-sided pyramid, or can be derived from any of these 

 figures The rhomboid, the four-sided square-based 

 pyramid, and the four- sided oblique-based pyramid, 

 are forms which cannot by any means be derived from 

 each other ; hence the four groups of simple forms, as 

 well as their combinations, must each be altogether 

 distinct from the rest ; and hence arises a correct and 

 natural division of all possible crystallizations, which 

 is of great utility in mineralogy. An interesting 

 sketch of Mohs' Views in Crystallography, is given in 

 the Edinburgh Philosophical Journal, vol. iii. p. 154". 

 Fig. 10. Plate CCCXCVI. represents the rhomboid; 

 Fig. 30, Pyramid milk square bate; Fig. 6, Oblique 

 four-sided prism ; Fig. 3, Hexahedron or cube ; Fig. 

 <), Tclrahtdrou ; Fig. 2, Pentagonal Dodecahedron ; 

 and Fig. 35, the Tetragonal icostietrahedron. 



The nomenclature of the species used is nearly that 

 of Mohs, and is founded on the primitive forms of the 

 minerals, on the nature of their cleavage, or on the po- 

 sition of the bevelment. 



According to Mohs, as already mentioned, all the re- 

 gular forms in the mineral kingdom are reducible to 

 gome one of four great systems or groups, named 

 Uhomboidal, Pyramidal, Prismatical, and Hexahedral 

 or tessular, incluJing octahedron, cube octahedron, 

 rhomboidal dodecahedron, &c. Thus, in the genus 

 Corundum, there are three species in which the primi- 

 tive forms are the octahedron, rhomboid, and prism ; 

 and hence these are named, octahedral corundum, rhom- 

 boidal corundum, and prismatic corundum. In the 

 genus Zeolite, there are seven species; one of these is 

 named Prismatoidal zeolite, because the cleavage is 

 prismatoidal ; another is named Axifrangible, because 

 one of its most striking characters is its axifrangible 

 cleavage. In the genus Augite, one species is named 

 Oblique-edged augite, because the edge formed by the 

 meeting of the bevelling planes, on the extremities of 

 the crystal, is placed obliquely to the axis of the prism; 

 .mother species is named Straight-edged augite, because 

 the ed^e formed by the bevelling planes on the extre- 

 mity, is straight or perpendicular to the axis of the 

 prism. 



In the generic characters, the number of axes of the 

 crystals is given. But it will be inquired, what is here 

 understood by axis. When the section of a simple 

 figure, as a rhomboid or cube, affords, by means of a 

 plane which does not pass through its centre, a regular, 

 or equi-angular or equi-lateral figure, or one in which 



such a figure can be inscribed, the straight line, which Oryctogno- 



stands perpendicular on the middle point of the figure, t- 



and passes through the centre of the figure, is an axis. **~* "Y"""'' 

 If we take a hexahedron, and place it in such a situa- 

 tion that two only of its planes are horizontal, and the 

 others vertical, every section of it, with a horizontal 

 plane, will afford a square ; and the vertical line, which 

 stands perpendicular on the middle point of the square, 

 and passes through the centre of the figure itself, will 

 be an axis. Bring the same hexahedron in such a si- 

 tuation, that one of its solid angles is above, and ano- 

 ther vertically under it. The section with a horizontal 

 plane will be an equilateral triangle or equi-angular 

 hexagon, and the straight line perpendicular on the 

 middle point of this plane, and through the centre of 

 the figure, an axis. Lastly, If we place the hexahe- 

 dron in such a situation, that four of its edges are ho- 

 rizontal, and the others are equally inclined towards 

 the horizontal plane ; all the sections but two will be 

 longish rectangles, and the straight line, perpendicular 

 on the middle point, and through the centre of the 

 figure is an axis. The kind of axis is determined by 

 the figure of the section, and one and the same figure 

 may contain not only many, but also axes of different 

 kinds. That axis in which the form of the section is 

 triangular, or in which a triangle can be inscribed by 

 connecting some of its angles by straight lines, is named 

 a rhomboidal axis, because it occurs in the rhomboid ; 

 when the form of the section is a square, the axis is 

 named pyramidal, because it occurs in the pyramid with 

 square bases; and when the form of the section is 

 rhomboidal, the axis is named prismatic, because it oc- 

 curs in the oblique double four-sided prism, which is a 

 member of the prismatic series. In the Tabular View, 

 the Diamond is said to have many axes, because its pri- 

 mitive figure, the octahedron, has rhomboidal axes that 

 pass through the centre of the planes, pyramidal axes 

 that pass through the angles, and six subordinate axes 

 that pass through the middle point of the edges. Zir- 

 con is said to have one axis, because its primitive figure 

 belongs to the pyramidal system, in which there is only- 

 one principal axis. Topaz has three axes, because it 

 belongs to the prismatic series, in which there are three 

 principal axes. 



II. SYSTEM OF ARRANGEMENT. 



Three modes of arranging simple minerals have been System of 

 employed by naturalists : in the first, they are arranged arrange- 

 according to their chemical composition ; in the se- ment. 

 cond, in conformity with their external characters ; and 

 in the third, both methods are conjoined. The pure che- 

 mical method is desirable in a system of chemistry : the 

 mixed method, from its unsatisfactory nature, should be 

 banished from mineralogical science; while that found- 

 ed on external characters, or what has been called the Na- 

 tural History Method, ought to be adopted as the only 

 true and scientific plan for the purposes of natural his- 

 tory and practical utility. The natural history method 

 is followed in the present article, and the arrange- 

 ment is that of Professor Mohs of Freyberg. Dr. 

 Walker, the former Professor of Natural History in the 

 University of Edinburgh, taught mineralogy according 

 to the natural history method. Professor Jameson, 

 his successor, has frequently done the same ; and we 

 find, from the introduction to the third edition of his 

 System of Mineralogy, that he now abandons the mixed 

 method, and adopts the natural history method. 



The following is a tabular view of the plan of ar- 



