448 



MINERALOGY. 



0/yctogno- ments of eight-sided prisms, and so on. Mr. Daniell 

 .,**''. nas shewn, in a satisfactory way, that these emboss- 

 "*^ ir ments are not formed by the crystallization of that 

 portion of the salt which has been dissolved ; but 

 that they are brought into view by the unequal so- 

 lution of the lump of salt subjected to the action of 

 the water. Hence it follows, that all these appa- 

 rently amorphous masses are in reality composed of 

 crystals, though such a structure cannot be distin- 

 guished by the eye previous to this natural dissection 

 of it. The same crystalline structure was developed 

 when calcareous-spar, strontian and witherite, were 

 acted on by vinegar. Bismuth, antimony and nickel, 

 treated with very dilute nitric acid, likewise exhibit- 

 ed a crystallized structure. From these experiments 

 we may infer, with considerable probability, that 

 the structure of most minerals is in reality crystalli- 

 zed, even when they appear massive; an inference 

 which leads to the highly important conclusion, that, 

 on a general view, a great portion of the crust of the 

 earth is more or less of a crystalline nature. 



B. Disseminated, is from the size of a hazel-nut until it 

 is scarcely visible, and its dimensions in length, 

 breadth, and thickness are nearly alike. It is imbed- 

 ded, and is intermixed with the inclosing mineral at 

 the line of junction. It is divided into 



a. Coarsely disseminated, which is from the size of a 

 hazel-nut to that of a pea. Examples, Copper- 

 pyrites and brown-spar. 



& Minutely disseminated, from the size of a pea to 

 that of a millet-seed. Example, Tinstone in gra- 

 nular quartz. 



c. Finely disseminated, from the size of a millet-seed 

 until it is scarcely visible. Example, Brittle sil- 

 ver glance in brown spar. 



C. In angular pieces. Minerals having an angular 

 shape, in which the length, breadth and thickness 

 are nearly alike, which are found loose, or slightly 

 imbedded, and without any intermixture with the 

 inclosing mineral at the line of junction, and from 

 the size of a hazel-nut and upwards, are said to oc- 

 cur in angular pieces. It is distinguished from the 

 massive by its occurring either loose, or not inter- 

 mixed with the basis at the line of junction. Of this 

 external shape there are two kinds. 



a. Sharp cornered, as in quartz and calcedony. 



b. Blunt cornered, as in common opal. 



D. In grains. Minerals having a roundish form, and 

 imbedded or loose, and not much larger than a hazel- 

 nut, are said to occur in grains. This shape is distin- 

 guished 



a. With regard to size, into 



tc. Large, that is, when they are from the size of 

 a hazel-nut to that of a pea. Examples, Mea- 

 dow ore and precious garnet. 



. Coarse, from the size of a pea to that of a hemp- 

 seed. Example, Pyrope. 



y. Small, from the size of a hemp-seed to that of a 

 millet-seed. Examples, Precious garnet, pyrope 

 and iron-sand. 



2. Fine, from the size of a millet-seed until it be- 

 comes nearly undistinguishable. Example, Pla- 

 tina. 



The grains are further distinguishable 



b. With regard to the exacter determination of the 

 shape into 



. Angular grains, as in iron sand. 



/}. Flattish grains, as in platina and gold. 



y. Roundish grains, as in pyrope and precious gar- Oryciogno- 



net. sj- 



c. With regard to connection with other minerals. ^T" 

 . In loose grains. 

 ft. In imbedded grains. 

 /. In superimposed grains. 



E. In plates. Minerals which occur in external shapes 

 whose length and breadth are great in comparison of 

 their thickness, in which the thickness is not equal 

 throughout, and is so considerable, as to allow the 

 fracture to be distinguished, are said to occur in 

 plates. The maximum thickness of plates is half an 

 inch. Example, red silver. 



F. In membranes or flakes. This shape is distinguish, 

 ed from the former by its thinness, as it never great- 

 ly exceeds the thickness of common paper, and the 

 fracture cannot be seen. Example, iron-pyrites. 



II. Particular External Shape. 



Particular external shapes differ from the common Particular 

 external shapes, in bearing a resemblance to natural or external 

 artificial bodies, and in being far more characteristic sha P e< 

 and varied in their aspect. They are called particular, 

 because they are not so common among minerals, as 

 the common external shapes. There are four different 

 sets, entitled, longish, roundish, flat, and cavernous. 

 Each of these sets have their subordinate kinds, which 

 we shall now describe. 



A. Longish Particular External Shapes. 



a. Dentiform, adheres by its thick extremity, and be- 

 comes gradually thinner, incurvated, and at length 

 terminates in a free point, so that it resembles a 

 canine tooth, whence its name. Its length is from 

 a quarter of an inch to a foot. It is one of the 

 rarer kinds of external shapes, and is peculiar to 

 certain metals. Example, Native silver. 



b. Filiform, adheres by its thicker extremity, and ter- 

 minates by an almost imperceptible diminution of 

 thickness, and is usually curved in different direc- 

 tions. It is thinner and longer than the dentiform. 

 Example, Native silver. 



c. Capillary. W r hen the filiform becomes longer and 

 thinner, it forms the capillary. It is generally much 

 entangled, and sometimes the threads are so near 

 each other that it passes into the compact. Ex- 

 ample, Native silver. 



d. Reticulated is composed of many straight threads, 

 which are sometimes parallel and sometimes meet 

 each other at right angles, and form a net-like 

 shape. The whole is a series of minute crystals, 

 and is distinguished from the capillary by its 

 threads being always straight. Example, Native 

 silver. 



e. Dendritic. In this external shape we can observe 

 a trunk, branches, and twigs, which are distin- 

 guished from each other by their thickness, the 

 trunk being the thickest. Examples", Native cop. 

 per and brown hematite. 



f. Coralluidal or corallijorm. When two or three 

 branches, having rounded or pointed extremitiesi 

 proceed from one stem, the coralloidal external 

 shape is formed. There are usually many stems 

 together. From its resemblance to coral, it is de 

 nominated Coralloidal. The variety of arragonite, 

 called flos Jerri, is an excellent example of this 

 kind of particular external shape. 



