Carbonate of Lime

So early as tab. 20 I introduced to observation the primitive variety of Carbonate of Lime with the beginning of the metastatic bevillings, (if I may so term them to make them familiar,) and the rounding faces leading to the equiaxe, or those rounding faces which replace the terminal edges. It will now be convenient to refer to them to understand the present crystals, the faces of which are so enlarged as to become more difficult for the beginner to comprehend.

The figure is from a large and rare specimen, brought among others long since from Dufton in Westmoreland. So large a crystal without modifications has, perhaps, never been observed^*. This, like the above-mentioned much smaller specimens, has the rounding equiaxe so extravagant as to disguise it, and the other faces also much broader, and even additional truncations on the lateral edges. Another circumstance which gives it a more remarkable appearance is, that the superimposed nuclei which form the rounding faces have extended them, so as to occupy the parts of the rhomb between the apex and longer diagonals, forming a considerable elevation; and this has happened in several specimens,—see the right hand geometrical figure. The whole seems a trio composed of a rhomb and two equiaxed crystals regularly coalesced. The upper specimen, however, has a large rounding equiaxed crystal on the upper obtuse end, and on the lower termination a smaller one, with nearly the proper flatter faces. The faces on which the equiaxed crystal terminates, are joined to it curiously by three sides of a prism, irregularly and raggedly rounding, placed upon the diagonal above mentioned. The outline is naturally confused, but I hope will be understood, as it is a curious instance of the regularity attending crystallization, however irregular it at first appears.