THE 



MONTHLY MICROSCOPICAL JOURNAL. 



SEPTEMBER 1, 1873. 



I. — On Organic Bodies in Fire Ojpah 

 By Henry J. Slack, F.G.S., Sec. E.M.S. 



(Taken as read be/ore the Eoyal Microscopical Society.) 

 Plate XXVII. (Lower portion). 



Some time since I was indebted to W. G-. Lettsom, Esq., for a slice 

 of fire opal from Zimapan, in the centre of the Northern Mining 

 District of Mexico, in which he told me I should find a number of 

 crystals and other curious bodies. 



The crystals were scattered about in various positions, and were 

 most numerous in portions of the slide that did not contain many 

 organic-looking objects. I am told they have been recognized as 

 Tridimite. They are, as Figs. 7 and 8, PI. XXVII., show, hexagonal, 

 and, according to Von Eath's analyses, Tridimite is composed of 

 sihca with a little oxyd of iron, alumina, and magnesia. The fol- 

 lowing closely-corresponding analyses are taken from Poggendorff's 

 Annalen : * — ' 



Silica acid 96'! .. 95'5 



Ferric oxyd .. .. .. .. 1-9 .. I-? 



Alumina and magnesia j .. . . 1 ' 3 . . 1*2 



Loss ".. .. 0-66 .. 0-66 



99-96 99-96 



The opal slice exhibited several cracks and flaws, readily dis- 

 tinguishable with almost any illumination, but there were numbers 

 of objects, sometimes scattered irregularly and sometimes grouped 

 together, that did not present either aspect, but which might be 

 taken for minute veins filled with a whitish material. Most of the 

 aggregations had a more or less cylindrical appearance, such as 

 might be imitated by drawing a number of curved lines at approxi- 

 mately equal distances, and leaving irregular gaps at the bottom. 

 These might have been taken for purely mineral formations, and 

 indeed were so by some experienced mineralogists ; but in parts a 

 more organic character could be noticed, and by diligent search 

 throughout the slide and comparison of various objects, it appeared 

 that the complex and unintelligible forms were composed of simpler 

 ones, more or less injured, and crushed together. Fig. 1 represents 



* cxxxv., 1868, pp. 437 454. 

 VOL. X. K. 



