MINERALOGY. 



463 



In several minerals, two varieties of distinct concre- 

 tions, or different sizes of the same variety, occur toge- 

 ther, either the one including the other, or the one tra- 

 versing the other. Thus some varieties of schorl are 

 composed of large granular concretions, and these, 

 again, are formed of prismatic concretions ; some va- 

 rieties of straight lamellar heavy-spar, are composed of 

 large granular concretions, and these, again, of thin 

 and straight lamellar concretions ; and peastone af- 

 fords another example of the same kind of structure, 

 it being composed of round granular concretions, and 

 each of these of concentric curved lamellar concre- 

 tions. 



In other minerals we observe different kinds of dis- 

 tinct concretions intersecting each other, as in ame- 

 thyst, where curved lamellar concretions intersect 

 prismatic concretions, and in red and brown hematite, 

 where granular concretions are intersected by lamellar 

 concretions. 



VIII. The Surface of the Distinct Concretion*. 



Distinct concretions exhibit the following varieties of 

 mrface. 



Smooth, at in hematite and heavy-spar ; rough, at in 

 day ironstone ; ttreaked, which is either longitudinally 

 ttreaked, as in schorl, obliquely streaked, as in calca- 

 reous-spar, or transversely ttreaked, at in amethyst ; 

 uneven, at in brown blende. 



IX. The Luttre of the Distinct Concretions. 



It is determined in the tame manner as the external 

 liutre. 



V. THE GENERAL ASPECT. 



Under this head we include those characters for the 

 sight which are observed in minerals in general. These 

 are, the Transparency, the Streak, and the Soiling. 



X. The Traniparency. 



This character presents the five following degrees : 

 1. When a mineral, either in thick or thin pieces, al- 

 lows the rayt of light to pass through it so completely 

 that we can clearly distinguish objects placed behind 

 it, it it said to be transparent It is either sim/ily 

 transparent, that is, when the body seen through it 

 appears single, as in mica and selenite ; or duplicating, 

 when the body seen through it appears double, as in 

 calcareous spar. 



The dUtuncc of the two images is in proportion to 

 the thickness of the specimens, and is very incon- 

 siderable in thin pieces. The duplicating property, 

 or double refracting power of calcareous spar, is ob- 

 served by looking through two parallel planes ; but 

 in some other minerals it is observed by looking 

 through two planes obliquely inclined on each other. 

 J. Srmi-lransparent ; when objects can be discerned 

 only through a thin piece, and then always appear at 

 if teen through a cloud. It is the least frequent va- 

 riety of this character, and occurs most frequently in 

 siliceous minerals. Examples, calcedony, common 

 and precious opal, and carnelian. 



3. Translucent. When the ray? of light penetrate into 

 the mineral and illuminate it, but objects cannot be 

 observed either through thick or thin pieces, it it 

 aid to be translucent Examples, pitchstone, quarts, 

 granular limestone, and massive fluorspar. 



4. Translucent on lite edges. When light shines through 

 the thinnest edges and comers, or when the edget are 



illuminated in the same degree as the whole mineral Oryctogno- 

 in the immediately preceding variety of transparen- _? 

 cy, it is said to be translucent on the edges. Exam- ^""Y '"* 

 pies, homstone, heliotrope, and compact limestone. 

 5. Opaque. When even on the thinnest edges of a mi- 

 neral no light shines through, it is said to be opaque, 

 at in chalk and coal. 



XI. The Opalescence. 



Some minerals, when held in particular directions, re- The opaki- 

 flect from single spots in their interior a coloured shin- cence * 

 ing lustre, and this is what is understood by opalescence. 



It is distinguished into 



A. Common or simple opaletcence, when the lustre ap- 

 pears massive, in undivided rays, as in cat's-eye, and 

 chrysoberyl. 



B. Stellular opaletcence, when the lustre appears in six 

 rays, or in the form of a star, as in the variety of 

 sapphire, named from that circumstance s.tar sapphire. 

 This phenomenon occurs principally in translucent 

 minerals. 



XII. The Streak. 







By the streak, we understand the appearance which Tl >e streak, 

 minerals exhibit when scratched or rubbed with a hard 

 Ixxly, asa knife or steel. In some instances the colour 

 of the mineral is changed ; in others the lustre, and 

 frequently neither colour nor lustre are altered. 



The streak, 



a. In regard to colour, is either 



. Similar to that of the mineral, as in chalk and 

 magnetic ironstone ; or 



ft. Diitimilar, as in specular iron-ore or iron-glance, 

 which has a steel-grey colour, but affords a cher- 

 ry-red streak ; wolfram, which has a greyish 

 black colour, but a brownish red streak ; and 

 red orpiment, which has an aurora red colour, 

 but affords an orange-yellow streak. 



b. In regard to luitrc, it remains 

 M. Unchanged, as in chalk. 



f. It incretiiid in intensity, Of a shining or glisten- 

 ing lustre appeart in minerals that otherwise hare 

 none. Thus steatite, which is sometimes glim- 

 mering, becomes shining in the streak ; and pot- 

 ter's clay, fuller's earth, and black and brown 

 cobalt ochres, which have no lustre, become 

 glistening or shining in the streak. 



c. It diminished in in/entity, or altogether destroyed. 

 Thus, grey antimony-ore loses its lustre in the 

 streak. 



XIII. The Soiling or Colouring. 



When a mineral taken between the fingers, or drawn Th foiling 

 across another body, leaves some particles, or a trace, or colour- . 

 it is said to sail or colour. ing- 



It is a character which occurs but in few minerals, 

 and only in those which are soft and very soft. Mine- 

 rals are said to 

 \. Soil, either 



A. Strongly, as chalk, drawing-slate, and reddle. 



B. Slightly, as graphite ; or 



2. Do not toil, as molylxlena. 



3. Write, as chalk, graphite, reddle, molybdena, and 

 black chalk or drawing-slate. 



Having now explained the External Characters which 

 are observable by the sight, we proceed to describe 

 those which are made known to us by the aenset of 

 Touch and Hearing. 



