Royal Society, 139 



properties. The considerable size of these topazes renders them 

 interesting as English specimens, those of Cornwall and the 

 Mourne Mountains, Ireland, being generally very minute, I 

 believe. 



Silex.—YYom the table of a mineral dealer on Clifton Downs 

 I lately obtained a portion of a large ironstone nodule, coated 

 internally with quartz, which is concealed under an accumulation 

 of siliceous matter, whose form is unknown to me, but possesses 

 a characteristic structure, having much the appearance of antho- 

 phyllite in this respect, a kind of indistinct radiating texture, 

 composed of extremely fine laminae in groups, and of a pinkish 

 hue. It is composed almost wholly of pure silex, with traces of 

 iron and lime, and possibly alumina, which I have not attempted 

 to separate from the iron, the former however so much in excess 

 as to render it probable that their presence is either accidental 

 or a mechanical admixture. The large nodule in which this is 

 contained is said to have been brought from the summit of 

 Broadfield Down, Somersetshire, where trap rocks make their 

 appearance at no great distance. The quartz is so concealed by 

 it, that it looks as though the former had been changed into it, 

 or its crystallization suspended by some alteration of heat or 

 pressure, or other cause, so that the elements have arranged 

 themselves in a new form. 



Celestine. — The beautiful and unique variety of this mineral, 

 found at Pyle Hill, near Bristol, presenting fine crystals of the 

 primitive rhombic prism with single replacements on the alter- 

 nate solid angles, so well investigated and described by Mr. 

 William Sanders, of Bristol, some years back, has not as yet, 

 that I know of, been noticed in any of the manuals. Some of 

 the smaller crystals, though retaining the general rhombic form, 

 show minute replacements in addition to the principal one on 

 the alternate solid angle, whose symbol it would be interesting 

 to determine, and of which I have preserved specimens. 



XXI. Proceedings of Learned Societies, 



ROYAL SOCIETY. 



[Continued from p. 74.] 

 May 18, 1854.— The Earl of Rosse, President, in the Chair. 

 HE following paper was read : — " On some conclusions derived 



T 



from the observations of the Magnetic Declination at the Ob- 

 servatory of St. Helena. By Colonel Edward Sabine, R.A., V.P.R.S. 

 The author commences with the following preliminary remarks : — 

 •* The part taken by the Royal Society in promoting, by its influence 

 with Government, the establishment of the Colonial Magnetic Ob- 

 servatories, and in drawing up instructions for the guidance of those 



