105 



tefnal, and-, if -I be^^lfeWed the expression, interna], 'are 

 all curves; its form is round, a?nd it is- fconipidsed (^f concen- 

 trie spheres, like the pellicles of an ollioii.'JfiJu i 



This variety Mr. St. Fond ttmself^ idiiMts^ not' to be of 

 volcanic origin. He says, (Min.des Yolc. page 46,)- it must 

 hme taken this configuration naturalii/. -ii^ rflode' of 'Arrange- 

 ment, in the places where it is found, sielems still more ex'^ 

 tcaordinary. It is generally imbedded' -in'^h indurated ba- 

 saltic paste; in INIr. St. Fond's language, incorporee et eii" 

 c(tgtrc6 dans des m((ssifs de'.^bdsatte'^tfot'rfi'dii In this istate, it 

 is sometimes built in the fdi-in of a wall/ of which the glo- 

 r.. '.'V.'"-"' ' ■ . bukr 



^-!iil/i .il/i him i'jniuhd jjIj Jno 



their insiJes sloped away, in an hyperbolic jCUrve; while the grooves, in the 

 lower part of each joint', adapted to receive these, with similar curvature, 

 added to the former, make twice as many curve surfaces, as the figure has 

 angles. The concave and convex bases add two m.ore; but, by Sir Tor- 

 bern Bergman's definition, crystals are bounded by plain surfaces. 



These facts cannot be exhibited in distinct 'joints; for, the cohesion is so 

 Strong, that the ascending pyramids invariably break- off, as the joints nrh 

 separated from the pillar. It . is ,iilip prcgecting fraqture'that 'remains, which 

 gives the joint the appearan9^[of a wz^j-pi ci^^pn,,^s- was observed by the 

 early writers on the suijject. . ' ' ' 



The destruction of these "asbehding pyramids, makes the' separate joint 

 totally different from what it was, when existing in the perfect pillar. 



To illustrate all this, I give a drawing of two pillars: one, as it appears, 

 when long exposed to the air, which acts principally upon the joints; while 

 the dilatation and contraction, from heat and cold, loosens the pyramids, 

 and separates them from the pilJirif (faofinoi C1R \')d} 



The second pillar exactly represents the state in which they appear; 

 where the mass is lately quarried into, and the air has not had time to 

 operate. i' • ■ .. i-'j'i,:;;. : If.. ... ix 



I add some joint*'?n' their natural state; 'This nicety 'of cbtistructioii 'abates, 

 as, the pillars graduate through imperfeqtiop, to an amorphous mass; j-gt 

 occasional traces of it are long observable. 



