456 



MINERALOGY. 



Oryctogno- impressed their form on the basis, and portions of 



_ Y _' the basis are sometimes contained in the crystals, 



and the crystals at their lines of junction, are occa- 

 sionally intermixed with the basis. Examples of 

 this we have in garnets imbedded in serpentine, 

 or garnets in mica-slate. 



y. Superimposed. When crystals rest upon the sur- 

 face of another mineral, and are firmly attched to 

 it, they are said to be superimposed. No regular 

 planes occur at their point of attachment ; on the 

 contrary, they take the impression of the kind of 

 surface on which they rest. Hence it would ap- 

 pear, that probably they are often of posterior 

 origin to the basis on which they rest. 



The second distinction is into 

 B. Aggregated. Here there are two distinctions. 



a. Where a determinate number of crystals grow to- 

 gether in a determinate manner, and these differ, 



1. With respect to number. 



i. Pair-nise (twin crystals.) 



it. Three together (triple crystals.) 



iii. Four together (quadruple crystals.) 



2. With regard to the manner of their intersection. 

 i. Penetrating one another. 



ii. Intersecting one another. 



iii. Adhering to one another. 



Twin crystals are formed by two crystals pene- 

 trating, intersecting, or adhering to one an- 

 other. Of the Jirsl we have an example in 

 felspar, where they penetrate one another in 

 the direction of their thickness ; in gypsum, 

 where they penetrate one another in the di- 

 rection of their breadth ; and in calcareous- 

 spar, where they penetrate one another in 

 the direction of their length. Of the second 

 we have an example in cross-stone, where 

 the crystals intersect each other, and form a 

 kind of cross, and have a common axis ; and 

 of the third in spinel, where the crystals ad- 

 here only by some of their planes. 



Triple crystals, occur in spinel and calcareous* 

 spar. 



Quadruple crystals, occur rarely, as in tin-stone. 



b. Where there are many crystals together, but merely 

 simply aggregated ; and these are either, 1. On one 

 another ; 2. Side by side, or adhering laterally to one 

 another ; and, 3. Promiscuous. 



The _ first occurs principally in tessular crystals, as in 

 galena or lead-glance and fluor-spar. The se* 

 cond occurs in amethyst, where the pyramids or 

 prisms are parallel among themselves. The third 

 occurs principally in long and broad figures, as in 

 tables and prisms. We have examples of it in grey 

 ore of antimony, where very long and nearly needle- 

 shaped crystals cross one another in different direc- 

 tions ; also in tabular crystals, and this kind of ta- 

 bular aggregation has much resemblance to the 

 cellular external shape. 



c. Where there are many crystals together, but doubly 

 aggregated. 



This kind of aggregation is distinguished from the 

 foregoing by its forming groupes that exhibit shapes 

 resembling bodies in common life. 



i. Scopiform orfascicular. It isYomposed of a number Oryetogno- 

 of thin prismatic crystals, 'diverging from their *J- 

 point of attachment, and thus forming a kind of '^""Y"" 

 fasciculus or bundle. Examples, calcareous spar 

 and zeolite. 



ii. Manipular sheaf-like. Consists of a number of 

 crystals that diverge towards both ends, and are 

 narrower in the middle, thus resembling a sheaf. 

 It occurs in prismatic and tabular crystals. Ex- 

 amples, zeolite, calcareous spar, and prehnite. 



iii. Columnar. Consists of very long needle-shaped 

 prisms, many of which are connected together in 

 the direction of their length ; and these columnar 

 groupes sometimes cross one another in different 

 directions. Example, columnar heavy spar, and 

 white-lead-spar. 



iv. Pyramidal, is composed of many long prismatic 

 crystals that are parallel to one another, but of 

 which those in the middle are the highest, and the 

 others decline on all sides, from the central one. 

 Example, calcareous spar. 



v. Bud-like, is composed of low (generally) six-sided 

 pyramids, one of which is usually situated in the 

 middle, and is surrounded by a number of others, 

 whose extremities are directed towards one another. 

 Here also many groupes occur together. Example, 

 quartz. 



vi. Amygdaloidal is formed by tables disposed around 

 each other, in such a manner as to form an amyg- 

 daloidal shape. Example, straight lamellar heavy 

 spar. 



vii. Rose-like, is composed of very thin six-sided tables, 

 which are repeatedly curved, and so connected to- 

 gether that the groupe resembles a blown rose. It 

 occurs in the variety of calcareous spar called rose- 

 spar, from Joachimsthal. 



viii. Globular. Is composed of tables or cubes aggre- 

 gated into a globular shape. Examples, iron* 

 pyrites, and curved lamellar heavy spar. 



ix. In rows. When many crystals are superimposed 

 on each other, in a straight direction, like the pearls 

 in a necklace, they are said to be aggregated in 

 rows. The flat three-sided pyramids of calcareous 

 spar, and the octahedrons of silver-glance or sul- 

 phureted silver, afford examples of this kind of ag- 

 gregation. 



x. Scalarmise, in which many tessular crystals are 

 arranged like steps of a stair. Example, cubes of 

 corneous silver. 



IV. Extraneous External Shape. 



Extraneous external shapes of minerals are those Extraneoa* 

 derived from organic bodies. They are also named external 

 Petrifactions, and less properly Fossils. The particular shape, 

 study of these interesting forms belongs to geognosy, 

 as the oryctognost views them only in a general way. 

 In general, they are arranged in the order in which 

 the species are described in the natural history of orga- 

 nic bodies, and are divided into petrifactions from the 

 animal kingdom, and into those from the vegetable king- 

 dom*. 



* In describing petrifactions, with the view of a complete history of the species, a more regular and comprehensive arrangement ought ti- 

 be followed than that usually employed by naturalists. In a paper which Professor Jameson read before the Wernerian Society some years 

 ago, he proposed and adopted the following arrangement, in describing a petrifaction from Sicily. 1. Description of the external aspect 

 and internal structure. 2. Chemical characters, and chemical composition. 3. Geognostic situation. 4. Geographic situation. 5. Uses. 

 6. History, under which head he includes all that it known of the first discovery of the petrifaction, the names it may have had at dif- 

 ferent times, the different figures and descriptions of it published by authors, and other information of a miscellaneous nature. 



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