MINERALOGY. 



461 



cording to thickness, direction, perfection, and clea- 

 vage. 



a. Thiclcneu. 



a. Thick ilaty, as in alum-slate, flinty-slate, and 

 clinkstone. 



b. Thin tlaty, as in most of the varieties of clay- 

 slate. 



b. Direction. 



a. Straight slaty, as in common clay-slate. 



b. Curved tlaty, which is either, 



aa. Indeterminate curved tlaty, as in some va- 

 rieties of bituminous marl-slate. 



bb. Undulating curved tlaty, as in glossy alum- 

 slate. 



c. Perfection. 



a. Perfect tlaly, as in clay-slate. 



b. Imperfect Italy, as in common flinty-slate. 



d. Cleavage. 



a. Single cleavage, which is the usual variety in 

 clay-slate. 



b. Double cleavage, rare, as in clay -slate. 



2. Where several fracture*- occur at the tame lime, their 

 relative tituation mutt be obierved. 



A. One including the other. 



In some minerals there occurs a double frac- 

 ture, in which the one fracture is larger than the 

 other, and includes it ; the one, the larger frac- 

 ture, is named thefraclurt in the great ; the other, 

 the lesser, the fracture in the small; thus, whet- 

 slate has in the great a slaty fracture, but in the 

 small a splintery fracture. 



B. One traversing the other. 



In other minerals, where the fracture also is 

 double, but in which the length and breadth are 

 different, that fracture which is in the direction of 

 the length is named the loneitutlinal fracture ; the 

 other, in the direction of the breadth, the trans- 

 terse or crott fracture. Thus, in topaz, there is a 

 conchoidal longitudinal fracture, and a foliated 

 transverse or cross fracture. Hut in tessular crys- 

 tals, where the length and breadth are nearly 

 alike, we use, in place of the term longitudinal 

 fracture, principal fracture, and apply it to that 

 fracture which occurs the most frequent in break- 

 ing a mineral ; the other fracture, Mr crott frac- 

 ture. Thus in blende, the principal fracture is fo- 

 liated, with a sixfold cleavage ; but the crois frac- 

 ture is conchoidal ; and in drawing slate, the prin- 

 cipal fracture is slaty, and the cross fracture is 

 earthy. 



VI. The thape of the Fragment*. 



Tb thape Fragments are those shapes which arc formed when 

 f tb a mineral is so forcibly struck or split, that masses 

 fragment!, having surrounding fracture surfaces are separated 

 from it. 



The fragments are cither regular or irregular. 

 1. Regular fragment*, are inclosed in a certain num- 

 ber of regular planes, that meet under determinate 

 angles. They occur only in such minerals as have 

 a foliated fracture, with several cleavages. Each 

 cleavage in these regular fragments, forms two 

 opposite parallel planes, and the shape of the 

 fragment depends on the number of these planes, 

 and the magnitude of the angles under which they 

 meet. Minerals with a twofold cleavage, do not 

 afford perfect regular fragment*. The following 

 are the varieties of regular fragments : 



A. Cubic, which occur in minerals possessing a Oryctogno- 

 rectangular threefold cleavage, as galena or lead- *J 

 glance and rock-salt. SP Y"' 



B. Rhomboidal or oblique-angular, which occur in 

 minerals having a threefold cleavage, as calca- 

 reous-spar. When two cleavages intersect each 

 other obliquely, and are intersected rectangu- 

 larly by a third, the fragments are oblique-an- 

 gular in one direction, and rectangular in an- 

 other, as in felspar and selenite. In calcareous- 

 spar, the fragments are specular on every side; 

 but in felspar, owing to the imperfect third 

 cleavage, only on four sides. 



C. Trapezoidal. Occur in foliated coal. 



D. Telrahedral or three-tided pyramidal and octahe- 

 dral, occur in minerals having a fourfold clea- 

 vage, in which the folia meet under equal an- 

 gles, as in fluor-spar. Three and six sided pris- 

 matic fragments occur in minerals having a four- 

 fold cleavage, in which three of the cleavages 

 are placed under equal angles around a common 

 axis, and are rectangularly intersected by the 

 fourth, M in beryl. 



E. Dodecahedral. Fragments of this form occur 

 in minerals having a sixfold cleavage. Some- 

 times three of the cleavages are disposed around 

 an axis, and are obliquely intersected with other 

 three, as in blende ; in other instances, all the 

 six cleavages intersect each other under equal 

 hexagon angles, and terminate in an apex, form- 

 ing double tix-tidcd pyramidal fragments, as in 

 rock-crystal. 



. Irregular fragments. 



These have no regular form. They occur in mi- 

 nerals with a single cleavage, and in all the varie- 

 ties of compact fracture. The following are the 

 different varieties: 



A. Cuneiform, in which the breadth and thickness 

 re much less than the length, and gradually 

 and regularly diminish in magnitude from one 

 end to the other. It occurs in minerals pos- 

 sessing a scopiform radiated fracture, as Cornish 

 tin-ore, red-hematite, and radiated-zeolite. 



B. Splintery, in which the breadth and thickness 

 are less considerable than the length, but with- 

 out diminution of magnitude from one extre- 

 mity to the other. It occurs in minerals having 

 parallel fibrous and radiated fractures, as in 

 asbestus and bituminous wood. 



C. Tabular, in which the breadth and length are 

 more considerable than the thickness, and the 

 middle is frequently thicker than the sides, 

 which indeed are sometimes thin and sharp. It 

 occurs in minerals with a single cleavage, as 

 mica, also in slaty minerals, as clay-slate, and 

 there is occasionally a tendency to it in minerals 

 with a conchoidal fracture, as flint. 



D. Indeterminate angular, in which the length, 

 breadth, and thickness are in general nearly 

 alike, but the edges differ much in regard to 

 sharpness, which gives rise to the following 

 distinctions. 



a. I'ery sharp-edged, as in obsidian and rock- 

 crystal. 



b. Sharp-edged, as in common quartz, pitch- 

 stone, and jasper. 



c. liathrr sharp-edged, as in basalt and limestone. 

 <l. Rather blunt- edged, as in pumice and copper- 

 pyrites. 



