L I M E. 



9 the fummits become ftill more acute, an acute double llirec- 

 lided pyramid is formed, f. The acute double three-lidod 

 pyramid is fometimes truncated on the lateral gdgcs, fome- 

 times bevelled : in the latter cafe, when the bevelling planes 

 become fo large that the original ones are very fmall, or even 

 difappear, the refult is an acute double three-fided pyramid, 

 having its planes length-v.ife divided ; or it is a double fix- 

 fided pyramid, g. If the fummits of the double fix-fided 

 pyramid are deeply truncated, it gives rife to the fix-fided 

 table, having its terminal planes fet on alternately in oppo- 

 lite dircAions. 



Though the preceding defcription of the different modi- 

 fications of Calcareous fpar may poflefs the merit of fpeakin^ 

 to the eye, yet it can by no means fuperfede the details of 

 a llriclly cryilallographical inveftigation ; we therefore fub- 

 join a fiiort abftraA of the excellent clafUlication given by 

 coimt B nu-non.in the firll volume of his " Tratte de Mine- 

 ralogie" lately publithed. 



All the modifications of cryftallized carbonate of lime are 

 by this author divided into, i, prifmatic; 2, rhomboidal ; 

 and J, pyramidal modifications. 



I. Prl malic Modifications. 



1. Prifm from th: eigei of the hafc of the pr'tnuti-ve cryflal ; 

 or thefe edges intercepted each by a plane. — Of this, a 

 variety with very fnort prifm occurs in Cumberland and 

 Uerbyfhire. That with long prifm ( Chaux carbon, prifme, 

 Haiiv, pL 24. f. 10 ) is likewife found in Cumberland. 

 The variety in which the lateral planes form rhombf, fo that 

 the cryftal at firft fight has the appearance of the garnet 

 dodecahedron, is the Icarceft of this modification. 



2. Prifm from the fol'td angles of the lafe ; thefe angles of 

 the primitive cryilalbcinginterccpredeachby a plane. — The 

 chaux carb. ImitaLli, Haiiy, (ib. fig. 12.) belongs to this 

 modification. It is generally feen in combination with 

 others. Sometimes two of the planes of the pyramid of the 

 primitifle rhomboid enlarge at the expence of the third ; and 

 fometimes one of them caufes, in the fame manner, the two 

 others to difappear. Found in Cumberland, Dauphine, 

 and on the Hartz. 



3. Summit intercepted by a plane perpendicular to the axis. — 

 This modification feldom occurs in its finiple fiate. The 

 varieties in which this plane is of confiderable extent is called 

 chaux carb. liafee by Haiiy (pi. 23. fig. 8.) This is fre- 

 quently feen nnited with the preceding modifications; in 

 which cafe, if the new plane has caufed the j-'vramid of the 

 rhomboid entirely to difappear, the chaux carb. prifmatique, 

 Haiiy, (pi. 24. fig. 14.) is produced, the fineft groups of 

 which are found in Cumberland and on the Hart/. Some 

 of the lateral ])lanes of thefe prifnis frequently enlarge, at 

 the expence of the adjoining planes; fo that one, two, or 

 even three, entirely difappear. The cryflals of the regular 

 hexahedral prifmatic variety, from the Hartz, are not unfre- 

 quently feen with white opaque furface, and alfo fometimes 

 to include fimilar cryflals of fmaller diameter, which now 

 and then project above the terminal plane of the larger cryf- 

 tal. The prifm of this variety is often fo fhort as to repre.- 

 fent a thin fix-fided table. When both the fecond and the 

 firil modifications are united in the prifm, we have the chaux 

 carb. peridodicacdre of Haiiy (pi. 26. fig. 33.) 



II. Rkomloidal Modifications. 



A. Obtufe Rhomboids. Of thefe, Nos. 5, 6. 8, and 9, have 

 not before been noticed. 



4. Obtufe rhomboid of l\^° "LC)', and G^,\\'. — One of the 

 moft common modifications, and more frequently than all 

 the rell (except the preceding) combined with otjiec 



modifications.' It is produced by the edges of the pyranids 

 of the primitive rhomboid being replaced each by a plane 

 equally i-.iclining on thofe by which the edge itfelf is formed. 

 In its complete ftate, tliis modification is the chaux carb. 

 iquiaxe of H;iuy (pi. 23. fig. 2.), wiiich is much more fre- 

 quently met with than the different pali'ages of the primitive 

 rhomboid into this modification. The planes of this fourih 

 modification are often longitudinally itriated ; and when 

 they are arrived at tlieir limits, the llris run parallel to the 



fhorier diagonal of the rhomboidal planes This is found 



united principally with No. 2, reprefcnting various degrees 

 of elongation of the chaux carb. dodkacdre, Haiiy (pi. 24. 

 fig. j8.) : found alfo as twin cryftals in Derbythire ; with 

 No. 1, belonging to the chaux carb. bifur.itaire, Haiiy 

 (fig. 17.\ ch;tily from Cumberland (rare); with Nos. i 

 and 2, and witn Nos. 2 and 3, chaux carb. tqui-vaknte, 

 Haiiy (pi. 25'. fig. 28 ), both from Cumberland ; with 

 Nos. 2 and 3, and part of the primitive planes, whence it is 

 called chaux carb. Iriforme by Haiiy ifig. 26.). Tlie lall 

 mentioned variety is from the Hartz ; and both the fpecimcn 

 of Haiiy and that defcribed by M. de Bournon are remark- 

 able, for having part of the pyramid covered with cryftalline 

 matter, whicli appears to be depohted after the cryftal had 

 been completed, and is thus forming a paflage into the 

 regular hexahedral prifm. 



5. Very obtufe rhomboid of \\y 56', and 6i' 4'. — It is 

 produced by tlie edges of the primitive rhomboid being re- 

 placed each by a plane, which is inclined towards its ium- 

 mit. This has hitherto been found only in combination 

 with other modifications, viz. Nos. 2 and 3. The cryflals 

 are all from the Hartz, where red filver ore is fometimes 

 accompanied with them. The variety of calcareous fpar, 

 called en rofe, generally belongs to the complete rhomboid 

 of this modification, as alio m.ofl of thofe known by the 

 names of coxcomb and lenticular ip.ir; but they are feidom 

 determinable by the goniometer. 



6. Obtufe rhomboid 0/" 1 13 and G'f. — This is cafily con- 

 founded with the preceding modification. Count Bournon 

 has obferved it only in a few inllances, combined with the 

 planes of Nos. i, 2, and 36 {vide infra), in cryflals from 

 Derbyfliire and Cumberland. 



7. Obtufe rhomboid of 107" 3', and 72" 57'. — This is the 

 refult of a decrement of the lamina;, ilniilar to tljat which 

 produces the preceding modification ; but the planes thus 

 formed are more inchned on the bafe of the primitive rhom- 

 boid. To this is to be referred Haiiy's chaux carb. guadri- 

 rhomboidale. (Ann. du Muf. d'H. Nat. t. I. pi. 8. fig. 4.) 

 It is always obferved in combination with other modifica- 

 tions, fuch as with thofe of Nos. i, 2, and with thofe of 

 the primitive cryllal. They come from Dauphine and 

 Derbyfhire. Th.is modification has not yet been feen pet- 

 feCl ; though nearly fo, in a variety which has very narrow 

 planes of No. i . One variety from Derbyfhire, which has 

 the prifm of No. 2 combined with this modification, and 

 part of the planes of the primitive rhomboid, might be cafily 

 miilaken for prifmatic rock cryllal, of which it has fome- 

 times the tranfparency. 'J^he planes marked / in Haiiy's 

 chaux carb. relrcgrade, (pL 26. fig. 36.) belong to this 

 variety. 



8. Very obtifs rhomboid of 118' 34', and 61' 26'. — This 

 moll obtufe of a'l rhon;boids known to occur in calcareous 

 fpar, is the refult of a decrement of the cryftalline lamina?, 

 at the obtufe angles of the planes that form the fclid angle 

 of the fummit, which is thus replaced by three planes refling 

 on thofe of the primitive rhomboid. This modification, 

 which is fcarce, does not occur in its complete ftate ; befides 

 with planes of the primitive rhomboid, it has been obferved 



F z in 



