452 



MINERALOGY. 



PLAT* 



Oryctogno- altogether in the matrix ; diamond, zircon, and fluor- 

 *J- spar, are examples. The six-sided, or hexahedral pyra- 



T T""~' / ~""' mid, occurs single, as in Fig. 12. and double, as in Fig. 

 13. Examples of it occur in sapphire and calcareous 

 |j spar, red silver, white lead spar, quartz, and amethyst. 



- - - The e jght. 9 i(] ec j j s always double, and acuminated on 

 both extremities by four planes, as in Fig. 35. Ex- 

 amples of it occur in leucite, garnet, and silver-glance, 

 or sulphureted silver. 



The table has four, six, or eight terminal planes. 

 The <Arec-sided tables are mere varieties of some of 

 the other figures. The four-sided table, Fig. 15. 

 occurs frequently, as in heavy spar, white antimony, 

 and yellow lead spar. The six-sided table, Fig._ 17- 

 occur still more frequently : we have examples of it in 

 mica, calcareous spar, heavy spar, and native gold. The 

 eight-sided table occurs in heavy spar, and yellow lead 

 spar. 



S. Proportional Size of the Planes to one another. 



Proportion- This character is not of much importance. The 

 a! size of planes are either equilateral or unequal ; where they 

 the planes are unequal, they are either indeterminately or deter- 

 to one ano- minately unequal. The determinately unequal planes 

 tner> are alternately broad and narrow ; with two oppo- 



site planes broader ; with two opposite planes nar- 

 rower. 



In the hexahedron, dodecahedron, and icosahedron, 

 the planes are alike ; when any dissimilarity occurs, it 

 is merely accidental, and is therefore indeterminately 

 unequal. The three- sided prism shews only slight in- 

 determinate inequalities. The four-sided prism is not 

 always equilateral ; sometimes two opposite planes are 

 broader than the others, when the prism is said to be 

 broad, as in zeolite. The six-sided prism is almost al- 

 ways equilateral ; its varieties are generally accidental, 

 excepting the following, which are somewhat charac- 

 teristic. 1. The two opposite lateral planes broader 

 than the others, as in actynolite and heavy-spar. 2. The 

 planes alternately broader and narrower, as in calcare- 

 ous spar. The eight and nine sided prisms afford only 

 accidental or indeterminate varieties, as augate, topaz, 

 and tourmaline. 



In the pyramid, sometimes the two opposite plans 

 are larger than the others, when it is said to be broad. 

 The four-sided table is usually equilateral ; it has 

 sometimes, however, two opposite lateral planes longer 

 Fig. 16. than the others, as in Fig. 16. The six-sided table is 

 sometimes unequilateral, or two opposite planes are 

 Fig. 18. larger than the others, as in Fig. 18. ; and the eight- 

 sided table is usually longish. 



4. The Direction of the Planes or Faces. 



The direc- The direction of the planes or faces is either Rcctili- 



tion of the near or Curvilinear. 



planes or Rectilinear is the most common, and is the case with 



faces. almost all the fundamental figures. 



Curvilinear planes * differ partly by the position of 

 the curvature, which is either concave, as in fluor-spar ; 

 convex, as in diamond; concavo-convex, as in sparry 

 ironstone ; saddle-shaped, as in the lens ; they differ 

 also by the shape, being either spherical, as in brown 

 spar ; cylindrical, in which the convexity is either pa- 



rallel with the sides, as in iron pyrites, or parallel with Oryctogno- 

 the diagonal, as in fluor spar ; and conical, as in gyp- 

 sum, and probably also in galena or lead glance. ^V" 



5. Angles under which the Planes meet. 



The size of the angles formed by the meeting of the Angles un- 

 planes is determined either by means of an instru- der which 

 ment named Goniometer, or angle-measurer, or simply the planes 

 by ocular inspection. Several different kinds of gonio- meet - 

 meter have been contrived. They are described in the 

 article CRYSTALLOGRAPHY. 



The other mode of ascertaining the magnitude of 

 crystals, namely, by ocular inspection, without the aid 

 of the goniometer, is that practised by Werner. In 

 this way he determined the whole of the species in the 

 system. The following are the terms used by him 

 when describing the angles and edges of crystals. 



Several different kinds of angles occur in the funda- 

 mental figures : these are the angles of the lateral edges, 

 angles of the terminal edges, and the summit angles. 



1. The angles formed by the meeting of the lateral 

 planes, are named the angles of the lateral edges, or, to 

 shorten the description, simply lateral edges. Thus 

 we say acute and obtuse lateral edges, in place of 

 acute and obtuse angles formed by the meeting of 

 the lateral planes. The lateral edges are either 

 equiangular, or uncquiangular. In the icosahedron, 

 all the edges are equiangular. In the dodecahedron, 

 the edges are equiangular. The hexahedron is either 

 equiangular and also rectangular, or unequiangular 

 and oblique angular. The rectangular hexahedron is 

 named cube ; the oblique angular, rhomboid. In the 

 prism, the lateral edges are either equiangular or 

 unequiangular. The four-sided prism, with unequi- 

 angular lateral edges, is denominated an oblique 

 four-sided prism, Fig. 6. Plate CCCXCVI. In Fig . 6 . 

 the pyramid, the lateral edges are generally equi- 

 angular ; seldom unequiangular. The same is the 



case with the table; when the edges are unequian- 

 gular, we say the terminal planes are set obliquely on 

 the lateral planes. 



2. The terminal edges are formed by the meeting of 

 lateral and terminal planes. In the prism they are 

 generally equiangular, as in Fig. 6, Plate CCCXCVI. Fjg. <;. 

 and sometimes unequiangular, when we say that the 

 terminal planes are set obliquely on the lateral planes, 



as in Fig. 7, Plate CCCXCVI. They are always Fig- T- 

 equiangular in the pyramid. In the table they are 

 as in the lateral edges of the prism. 



3. The summit angle. It occurs only in the pyramid. 

 It is measured from plane to plane, or from plane to 

 edge. Werner determines it in degrees in the fol- 

 lowing manner. 



a. Extremely acute is from 1* to 30. 



6. Very acute, from 30 to 50. Example, Sapphire. 



c. Acute, from 50 to 70. Example, Calcareous spar. 



d. Rather acute, from 70 to 90. Example, Quartz. 



e. Rectangular 90. Example, Zircon. 



f. Rather obtuse, or rather flat, from 90 to 1 10. Ex- 

 ample, Honeystone. 



g. Obtuse or flat, from 110 to 130. Example, Calca- 

 reous spar. 



h. Very obtuse, or very flat, from 130 to 150. Ex- 

 ample, Tourmaline. 

 i. Extremely obtuse, or extremely flat, from 150 to 180. 



It is not geometrically correct to speak of curved planes ; yet, as the term plane is more generally used by mineralogists than face or 

 tide, we have not thought it necessary to make any alteration. 



