MINERALOGY. 



455 



The nag- 



r 

 ' 



larger at the expence of the original planet, which arc 



at length totally obliterated. 



These changes are produced either by truncation, 

 bevelment or acumination : the transition of the cube 

 into the octahedron, is an example of the first: the 

 transition of the octahedron into the icosahedron, by 

 the bevelment of the angles of the octahedron, of the 

 second ; and the third is exemplified by the transition 

 ul' the tetrahedron into the rhomboidal or garnet-dode- 

 cahedron, by the acumination of each of the angles of 

 the tetrahedron by three planes. 

 5. By the aggregation of crystals. Thus six-sided ta- 



bles heaped on one another form six-sided prisms. 



All the crystals that lie between two principal crys- 

 tals, and form the transition of the one into the other, 

 constitute what is called a transition-suite. These vary 

 in extent, and sometimes they form circles, 10 that 

 the last member of the suite passes into the first, or 

 they form a straight line, and diverge into numerous 

 branches. 



Those mineral species that occur crystallized, are 

 generally characterised by a particular suite of crys- 

 tals, which does not occur in the other species. There 

 are, however, mineral species very different from each 

 other in their external characters, in which we meet 

 with the same suite of crystals ; and still more fre- 

 quently do we meet with species that exhibit not the 

 whole suite of crystals of another species, but greater 

 or smaller portion of it. Thus there is an extensive 

 suite of crystals which extends from the icosahedrun, 

 through the dodecahedron, the cube, and the octahe- 

 dron, into the tetrahedron, part of which we sometimes 

 meet with in mineral species, but never the whole, only 

 larger or smaller portion of it. 



That member of the suite of crystals of a mineral 

 species, from which all the others originate or pro- 

 ceed, if ncmed the Fundamental Cri/ttaUizalioa. In 

 those suite* of crystal* which form circles, it is often 

 optional which of the figures we assume as the funda- 

 mental one ; and in those which are disposed in lines, 

 it is sometimes of little importance at which end we 

 begin our description. Still we always select that 

 crystallization which occurs the most frequently, and 

 the mot distinct in the species, and derive all the 

 others from it. 



In mineral species the crystals never appear isolated, 

 but form a kind of progression, and pass gradually in- 

 to each other. It follows from this important and 

 highly interesting fact, that when few crystals of a 

 species are known, probably all the intermediate mem- 

 bers of the series, which can be easily pointed out by 

 crystallography, and which have not been found, may 

 be expected to exist in nature, became the cause which 

 produced the one part of the series may also have form- 

 ed the others. 



C. The Magnitude of Crystalt. 



This character is useful, not only in the description 

 of varieties, but also in that of species, because in each 

 mineral ipecies the crystals appear to have a determi- 

 nate range of magnitude. We have here to attend to 

 the akiolule magnitude, and also to the relative magni- 

 tude of crystals. 



Crystals in regard to their Absolute magnitude, are 

 divided into 



M. 1'ncommonly lar^e, when the crystal is two feet 

 and upwards in length. The expression intimates 

 thut it is rare. Example, rock crystal. 



J. Firry large, from two feet to six inches in length. 



Examples, rock crystal, quartz, beryl, calcareous. Orytogo- 

 spar, and felspar. ^ 



y. Large, from six inches to two inches in length. 

 It is a very frequent size. Examples, lead- 

 glance, garnet, rock crystal, &c. 

 3. Middle-sized, from two inches to half an inch. 

 Examples of this magnitude are common, we shall 

 only mention galena or lead-glance, iron-pyrites, 

 fluor-spar, calcareous-spar, and garnet. 

 i. Small, from half an inch to the eighth of an inch. 



Examples, fluor-spar, calcareous-spar, &c. 

 . Very small, from the eighth of an inch in length, 

 until it is so minute as scarcely to be visible to 

 the naked eye. Examples, native silver, grey 

 copper-ore, spinel, &c. 



. Microscopic. When crystallized, but the form no 

 longer distinguishable by the naked eye. Exam- 

 ples, gold, galena or lead-glance, &c. 

 In determining the Relative magnitude of crystals, we 



use the following terms. 

 *. In the prism. 

 na. For the length. 

 Short at loir. 

 Long or high, 

 bb. For the breadth and thickness. 



Broad, when the breadth is greater than the 



thickness. 



Acicular or needle-shaped, when the prisms are 

 so thin that the planes are seen with diffi- 

 culty. 

 Capillary, when the planes of the crystals are 



no longer visible. 

 ft. In the pyramid. 

 aa. For the length. 

 Short or /ore. 

 Ijong or high. 



ko. For breadth and thickness. 

 Broad. 



Lance-shaped, allied to acicular. 

 y. In the table. 



aa. For the lencth and breadth. 



Loneish, when one dimension of the lateral 



planes is greater than the others. 

 46. For the thickness. 



Thick and thin. 



J. Crystals in which all the dimensions are nearly 

 alike, are named tctnlar. 



D. The Attachment of Crytlalt. 



Werner understands by attachment, the connection of Th n- 

 single crystals with massive minerals, and the aggrega- ttchmmt 

 tion of crystals together. According to the tabular of Cl 7 tta1 '- 

 view, the first distinction is into 

 A. Solitary; and this again into loose, imbedded, and 



superimposed. 

 *. Loose. Crystals are said to be loose, when they 



are not connected with any other mineral. 

 f. Imbedded. Crystals are said to be imbedded, 

 when they are completely enclosed in another mi- 

 neral. They are crystallized on all sides, or are 

 said to be all around crystallized, and must there- 

 fore have been formed at the same time with the 

 mineral in which they are imbedded. We cannot 

 conceive them to have been of anterior origin to 

 the basis in which they are contained : for, on this 

 supposition, we must conceive them to have re* 

 mained suspended in space until the basis was 

 formed around them. Nor can we admit them 

 to be of posterior origin, because the crystals have 



