t$8 Theory of Crvjlallifatwn. 



may- readify be conceived that the laminae, at the fame time 



that they decreafe towards their lower edges, ought, on the 



other hand, to increafe towards their ruperior edges, analo- 

 gous to a b, a j\ a g, See. (Jig. 4.) in fuch a manner that 

 the parts of the cryftal fituated towards thefe edges fhould be 

 always enveloped, and that the angles at the fummits of the 

 ihombufes mould remain contiguous to the axis. But this 

 is only an auxiliary variation, proper to fecond the effect of 

 the decreafe, which alone is fufficient to determine the form 

 of the dodeeacdron. 



Very ohtufe Calcareous Spar (Fig. 22). 



Sftath. .-. leaire rhomboidal tris obhts. Daubcnton. Tab. Mi- 

 ner, edit. 17923 p. 15, n . 2. Dc rifle, Cryflall. t. i. p. 504. 

 var. 2. 



Geomet.Characl. I'm lination of the rhbmbus n a d //to the 

 rhombus,.- i f ,7, T34 25 36''. Angles of the rhombus 

 11 a J 5 ; a or l> = ] \\ 18 $6 '; n or d' — 65" 4J'' 4". 



This variety, commonly called lenticular calcareous /par, 

 arifes from a decren fingle range on bulb fides of 



the edges a l\ ag, a /' fig. 2 }), znd ec } e d, e x, contiguous 

 to the fummits a, e, of the nucleus. An idea may he formed 

 of its ftraSure by comparing it with that or the dodecacdr m 

 whole plane's, form rhombufes [fig. ,'. and 12) originating 

 from the cube {Jig. 7) ; and by fuppofing e laminae, 



Inftead cr ; at the fame time on all the edges, de- 



creafe only towards thofe contiguous, three by three, to the 

 fingle C, and its oppofite. In that cafe the faces formed 

 will be reduced to fix, which, by prolonging themfetves ac- 

 cording to the law of continuity, io as to interfeel each 

 {Other, will compofe the furface of a rhomboid analogous to 

 that in qrtcftion ; except that it will have other angles oil 

 account of the cubical form of its integral molecular 



From what I Lave here faid it may be eafily cone i 

 that the diagonals drawn from a ig. 22), from a to g'^ 



front a to /'. ike, on the fecondary rhomboid, will be con- 

 9 founded 



