MINERALOGY. 



449 



g. Slalactitie. A mineral is said to possess a stalac- 

 titic external shape, when it consists of different 

 straight more or less lengthened rods, which are 

 thickest at their attachment, and become narrower 

 at their free extremity, which is rounded or point- 

 ed. Example, calc-sinter. 



h. Cylindrical consists of long, rounded, straight, im- 

 perforated, usually parallel rods, which are attach- 

 ed at both extremities, and are generally thick- 

 er at the extremities than the middle. The inter- 

 stices are either empty, or filled up with another 

 mineral. Examples, galena or lead-glance and 

 brown iron- ore. 



i. Tubifom consists of long, usually single, perfora- 

 ted tube*, which are somewhat longitudinally 

 knotty. Example, calc-sinter. 



*. CUviform is the reverse of stalactitic ; it is compo- 

 sed of club-shaped parallel rods, which adhere by 

 their thin extremities. Example, compact black 

 hematite. 



/. Fructicote. This external shape is formed when 

 many branches issue from a common stem and 

 meet together party wise, so that the whole when 

 viewed from above has a fructicose aspect, not un- 

 like the appearance of cauliflower. Examples, 

 calc-sinter and black hematite. 



B. Rounduh Particular External Shape*. 



a. Globular. Under this are comprehended. 



. Perfect globular or spherical, at in alum- slate 



and pisiform iron. 



/a. Imperfect globular, as in calcedony. 

 A. Ovoitial or Mptical. Example, rounded msssr* 



of quartz in puddingstone. 

 ). Spheroidal. When the spherical is compressed 



the spheroidal is formed. Example, Egyptian 



jasper. 

 . Amygdaloiaal. When the ovoidal is compressed 



in the direction of its length, the Amygdaloid*] 



i* formed. Examples, zeolite, calcareous-spar 



and green earth. 



b. Botryoidal consists of large segments of small balls, 

 which are regularly heaped together, and have 

 many interstices. It resembles grapes, whence its 

 name. Examples, hematite, and calcedony. 



c. Reniform consists of small segments of large balls, 

 which are so closely set together, that no intersti- 

 ces are formed. Example, calcedony. 



d. Tuberose. This shape consists of irregular round- 

 ish or longish elevations and depression). Exam- 

 ple, flint. 



e. futed like or liqviform. It consists of numerous 

 very flat rounded elevations, which are generally 

 depressed in the middle. The whole has a rough 

 and glimmering surface, and resembles the surface 

 of slowly cooled metal. Example, lead-glance. 



C. Fiat Particular External Sfiapei. 



a. Specular has on one side, seldom on two opposite 

 ides, a straight smooth shining surface. It occurs 

 in veins. Example, galena or lead-glance. 



b. In leaves. In this external shape there are thin 

 leaves, which are either irregularly curved, or are 

 straight, and have throughout the same thickness. 

 It is distinguished from the external shape in mem- 

 branes by the uniformity of its thickness, by its ir- 

 regular curvatures, its continuity, (the membra- 

 nous external shape being often interrupted,) and 

 its usual adherence by one extremity, shewing that 

 it is a kind of crystalline shoot It occurs frequent- 

 ly in native gold. 



VOL. xrv. PART u. 



D. Cavernous Particular External Shapes. Oryctogno- 

 a. Cellular. A mineral is said to be cellular, when ._/*_. 



it is composed of straight or bent tables, which *~*~~* 

 cross together in such a manner as to form empty 

 spaces or cells. Example, quartz. 

 6. Impresteil. That is, when one mineral shews 

 the impression of any particular or regular external 

 shape of another mineral. It borders on the cellu- 

 lar shape, and is formed when a newer mineral 

 is deposited over an older, the form of which it 

 assumes, and retains even after the impressing mi- 

 neral has been destroyed or removed. 

 a a. With imprestion* of crystalt. 



. Cubical, as in quartz or iron-pyrites, from 



fluor-spar. 

 3. Pyramidal, as in hornstone, originating from 



calcareous- spar. 

 y. Tabular, as in quartz, originating from heavy 



-pir. 



6 6. With imprettions of particular external shape). 

 m. Conical, in native arsenic. 

 $. Globular, in silver-glance or sulphureted sil- 

 ver? from red-silver-ore. 



/. Reitiform, in silver-glance or sulphureted sil- 

 ver, from red silver-ore. 



c. Perforated, consists of long vermicular cavities, 

 which occupy but an inconsiderable portion of the 

 mass, and terminate on the surface in small holes. 

 When the holes become very numerous it passes 

 into spongiform. Example, bog iron-ore. 



d. Corroded, A fossil is said to be corroded when it 

 is traversed with numerous hardly perceptible 

 roundish holes. The volume occupied by the 

 holes is nearly equal to that of the basis. It has 

 the appearance of wood which has been gnawed 

 by insects. Examples, quartz, galena or lead- 

 glance, and silver-glance or sulphureted silver. 



f. Amorphuutis composed of numerous roundish and 

 angular parts that form inequalities, between which 

 there are equally irregular hollows. The whole has 

 the appearance as if a number of small balls and 

 angular pieces were heaped on one another. Ex- 

 amples, silver-glance or sulphureted silver, and 

 meadow-ore. 



/. Veticular. When a mineral has distributed through 

 its interior many single, usually round, elliptical, 

 and spheroidal, also amygdaloidal, or irregular- 

 shaped cavities, it is said to be vesicular. The ca- 

 vities are usually less in volume than the solid part 

 of the mineral, and they are larger than the holes 

 or cavities in the corroded external shape. Ex- 

 amples, wacke and lava. 



E. Entangled Particular External shape. 



a. Ramose. It is composed of longish, angular, more 

 or less thick branches that are bent in different 

 directions, but in which no trunk or common stem 

 is to be observed. It probably originates from the 

 greater magnitude of the vesicles in the vesicular, 

 the vesicles breaking into each other. Examples, 

 meteoric- iron, silver-glance or sulphureted silver, 

 and native copper. 



III. Regular External Shape or Crytlallization. , 



Ever}* external shape, whose natural contour or out- Regular 

 line is composed of a determinate number of planes, external 

 which meet together in a determinate manner, is deno- shape or 

 minated a crystal. crjrstalliza. 



In describing crystals, we have to consider, A. Their tion '- 

 Ccnuiaenett; B. Their Sliape ; C. Their Magnitude ; 

 D. Their Attachment. 



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