Fig. 26. shews the passage into the secondary pyramid, which is complete in fig. 27.
Fig. 28. represents an elongated crystal; its elongation proceeds from a regular deposition of crystalline laminæ on one face of the upper and on one of the lower pyramid, and on the intermediate plane of the first modification. On the crystals represented by fig. 29. a deposition of lamina has taken place on two opposed planes of the first modification, gradually diminishing, so as to preserve the lengthened faces of the second modification perfect planes. On fig. 30. this species of deposition has taken place, after the crystal itself had been formed similar to that of fig. 28. Fig. 31. shews a crystal on which a regular deposition has taken place on two opposed faces of the upper and the two corresponding faces of the lower pyramid, so as to diminish two of the four triangular planes of each, and to give the other two the form of irregular hexahedral planes.
Third Modification.
This modification consists in a decrease on each apex of the primitive form, by which each is replaced by a quadrangular plane, perpendicular to the axis passing through the apices, Fig. 32. Pl. 16. The planes of this, though not uncommonly found in combination with those of other modifications, are rarely so well defined as to be depended on for accurate admeasurement, owing to an unevenness on their surfaces. I have not succeeded in finding crystals that have satisfactorily allowed the incidence of the planes of this modification with those of the primitive form.
Fourth Modification.
This modification consists in a decrease on each of the edges of the primitive crystal, formed by the meeting of the two pyramids
