188 Dr. Day's Mineralogical Notices, 



carbonate of lime, while the remaining portion is composed of 

 the cai'bonates of iron, magnesia, and manganese in varying pro- 

 portions. In various pieces which I have tried I have found 

 upwards of 20 per cent, of carbonate of magnesia, and never, I 

 believe, less than 10; while the reaction for manganese with 

 soda on platinum-foil, though interfered with by the tendency 

 of the mineral to grow dark under the blowpipe flame, is still 

 very evident. I do not pretend to the last degree of accuracy 

 in these examinations, and when the acknowledged difficulties of 

 the quantitative determination of magnesia are considered, this 

 will be excused. The analysis of carbonate of magnesia varies 

 much in different hands, as may be seen by reference to Brande ; 

 and I do not feel certain whether, in the combination with lime 

 and iron, the water should be regarded as an essential constituent. 

 On the principle of carbonate of magnesia entering as an iso- 

 morphic element into the union in question, I should think not. 

 By a reference to the analyses of foreign ankerite, it will be 

 noticed that while the carbonate of lime appears to be of nearly 

 uniform amount, agreeing almost exactly with my determination, 

 the quantities of carbonate of iron, magnesia and manganese, 

 which enter as isomorphic elements, are quite capricious, and all 

 my trials give results within the same limits. There cannot, 

 therefore, be any doubt of the close agreement of the species. 

 The quartz is sometimes singularly distorted by the unusual 

 enlargement of some of its planes, so as to create a momentary 

 difficulty in recognizing the form, and in other cases forms thin 

 plates cut at all angles to the axis. In one instance it forms a 

 thin hexagonal table, in which all the edges are bevelled, corre- 

 sponding to the situation of the twelve pyramidal planes, and all 

 the prism planes are reduced to thin lines of light when held so 

 as to catch the reflexion. 



Topaz. — Some time back a friend placed in my hands portions 

 of the rocks of Lundy Island ; and on looking at a small speci- 

 men composed of quartz crystals, I found three topazes among 

 the group, which I recognized by their crystalline form. They 

 can be traced quite through the body of the quartz crystals nearly 

 three-fourths of an inch long, but so that the latter have inter- 

 fered with their perfect development, indicating that they have 

 penetrated while the material of each was soft. Since this 

 another friend has brought from the island a detached crystal, 

 which he picked up there, exhibiting a curious fracture where it 

 has been torn from its matrix, which on examination with the 

 lens exhibits numerous small crystalline prominences, apparently 

 smaller individuals; This may, in fact, prove that it is not a 

 fracture proper, but such a termination as is often seen in tour- 

 malines, and which is thought to be connected with their electric 



