Crystallized Sulphate of Lime, or Gypsum

Shotover Hill, Oxfordshire, seems to afford the clearest and cleanest specimens of crystallized gypsum in the greatest variety: yet we find some varieties are rather local, as shall be shown hereafter. They are mostly found in a clayey gangue. The upper figure is what Haüy calls trapezienne. Although this would by extending the lateral faces, which might be easily done by piling plates on the summit, each smaller than the last, form an octaëdron; yet of a great many varieties which we have seen we have not observed this modification. The middle figure shows a very frequent variety from the same place, heightening towards an octaëdron; but seldom extending much further than this figure. They often have their angles a little irregular, so as not to meet: see the left hand corner near figures 1 and 2, also haveing other crystals sticking in them in different directions. If the laminæ are opened in the manner of a flaw or crack, when not too wide, they admit the prismatic rays: see the upper face of middle figure and middle face of lower figure. They are said to admit of double refraction by most mineralogical writers.

Fig. 1. is a darkish spot of clay or soil naturally in the subject and fig 2. is the same seen a little duller through the other face at the same time: but this is common to all transparent substances. It is somewhat curious that the clayey stripes or spots have a particular direction diagonally to the acute angles. Spec. Grav. 2.2642–2.3117. Kirw. and Haüy. They are laminated, the laminæ somewhat flexible; easily separated.

Hardness,—yields easily to the finger nail.

The bottom figure shows three crystals mixed together, and forming what is commonly called macle.