35 



TAB. CCCXXI. 



CALX carbonata, var. magnesiata. 

 Magnesian Carbonate of Lime with Iron, 



Div. 1. Crystallized. 

 Syn. Sidero-Calcite. Kirw. 1. 105. 



This curious Mineral was long since in Mr. W. Day's col- 

 lection, and I have had specimens sent me by P. Rash- 

 leigh, Esq. from Tincroft in the parish of St. John, Corn- 

 wall, as well as by another friend from that neighbourhood. 

 All are related to each other in the form of their crystals, 

 and instruct us in a series of modifications upon an acute 

 rhomb ; showing some very distinctly, and the manner of 

 accumulation necessary to form others : a very natural les- 

 son in Crystallography. In the upper figure are some mi- 

 nute perfect rhombs, resembling Haiiy's contrastante var. 

 of Carbonate of Lime — see the lot torn left hand figure ; 

 while others are larger with the three faces of the primitive 

 remaining on the acute points or pyramids. 



The middle figure shows a small truncation on the apex 

 of the rhomb, forming a triangular facet, which in some 

 Crystals is enlarged so as to cut off the obtuse lateral angles, 

 thus producing hexangular faces at each end. The next 

 magnified figure to that on the right hand shows an accu- 



VOL. IV. E 



