35 



TAB. CCCXXIL 



CALX carbonata, var. dodecaedra. 

 Dodeca'edrai Carbonate of Lime. 



Div. 1. Crystallized. 



Nature, ever instructing us with useful lessons, at first 

 sight often puzzles us. I had long since seen Quartzose 

 mixtures cast, as it were, over tabular Sulphate of Barytes, 

 forming a very accurate mould of them, small or large. 

 Such are commonly called Hacked Quartz, from the no- 

 tion that they resembled a substance partly chopped or 

 hacked with a hatchet or sharp instrument; "and their for- 

 mation was long left unaccounted for. 



It is not uncommon to find Ouartz moulds, if I may so 

 call them, of the Cubic Fluor*, on which it often forms 

 coats; and I have a very curious specimen of white semi- 

 opaque Quartz having formed over the Metastatic Carbonate 

 of Lime, from Polgooth in Cornwall, with which I was 

 favoured by Phil. Rashleigh, Esq.: but I had not seen Car- 

 bonate of Lime forming the mould of Sulphate of Barytes 

 till very lately. I received it from Alstone Moor in June, 

 I8O9. The specimen represented in the figure is crystallized 

 in the equiaxe form ; and I had another specimen sent at 

 the same time, which was cast over larger Sulphate of Ba- 

 rytes, crystallized in dodecaedrons with rhomboidal faces — 

 see tab. 128 — which is very rare: but a still rarer form is 



* Sulphuret of Iron often forms a mould from Cubic Fluor, &c. 

 E 2 



