CALCAREOUS-SPAR, OR CALC-SPAR. 163 planes, which are set on the obtuse lateral edges of the six-planed acumination. 3. When tlie planes of the three-planed acumination increase so much that those of the six-planed acu mination disappear, a six-sided prism is formed, flatly acuminated with three planes, which are set on the alternate lateral planes in an unconform- able position. 4. In some rare instances, the three-planed acumina tion is acute. In general, the angle of the acumi nation varies much in magnitude. When the prism becomes very low, there is formed a double three- sided pyramid, which must vary in its summit angle, according as the acumination is flat or acute. These prisms are often pyramidally aggregated. 6. The six-sided prism, acuminated with six planes, is often truncated on the apides : when these trun cating planes increase very much in magnitude, a six-sided prism is formed, in which the angles are more or less deeply truncated. 6. When the prism becomes very low, it tnaybe view- as an equiangular six-sided table, which is some times aggregated in a rose-like form. 7. Sometimes the six-sided prism is truncated on the lateral edges; III. Three-sidcd Pyramid. It originates from the prism N° 4. and has frequently remains of the prism on the common basis. It is gene- rally double, and the lateral planes of the one are set on the lateral edges of the other: Sometimes also single; and *'«* meet with nearly all the varieties that occur in the 1.2 aeuminations
