Limestone. 491 v 2. Six-fided Prifm. It occurs ufually with three alternate lateral planes broader than the others, and rather acutely acumi- n atedby fix planes which are fet on the lateral edges, a nd the acuminating planes meet alternately under filore obtufe angles. '' a. The fame prifm a fecond time flatly acuminated by three planes which are fet on the alternate obtule lateral edges of the firft acuminatiorr. b. When the planes of the fecond acumination en large themfelves fo much that thofe of the lirft entirely difappear, thence refults the fix- lided prifm flatly acuminated by^three planes, which are fet on the alternate and alternating lateral planes. c. The apex of the acumination is often more or lefs deeply truncated, which produces the fix- fided prifm in which the alternate and alter nating terminal angles are truncated. d. When the truncation of the apex become* fo large that all traces of the acumination difap pear, the perfe£t flx-fided prifm is formed. <r. When the prifm becomes lower, it pafles into the fix Tided table, which is often extremely thin.