Decmnpositioti of Iron Pyrites. 221 



this material, erected in the old burying ground at Peterhead, 

 sometime between 1785 and 1790, retains its lettering as sharp 

 and smooth as if only recently incised. Yet the stone is soft 

 enough to be easily cut with the knife. 



The cubes of pyrites bave resisted weathering so well that a 

 mere thin film of brown hydrous peroxide conceals the brassy 

 undecomposed sulphide from view. The slate is slightly stained 

 jellow round each cube or kernel of pyrites, but its general 

 smooth surface is not afllected. The lapse of nearly a century 

 [has produced scarcely any change upon this stone, while neigh- 

 l)oring tablets of white marble, 100 to 150 years old, present 

 rough granular surfaces and half effaced though still legible 

 anscriptions.'" Facts such as these show the necessity of the 

 •careful and exact determination of the nature of the contained 

 pyrites in all cases, before either the acceptance or rejection of 

 :a pyritiferous slate for roofing purposes. 



D. Process of Examination of Pyrites in regard to 

 Stability. The process of examination of a specimen of pyri- 

 tes "will be essentially the same, for any of the purposes just in- 

 dicated. The careful selection of the specimen is of the utmost 

 importance. Finely granular or even massive specimens will 

 will not be sufficient, in consequence of their probable enclosure 

 of impurities and cavities. It is highly desirable to obtain iso- 

 lated crystals, however minute, carefully separated from their 

 matrix, crushed to a coarse powder, freed from adhering iron- 

 ochre by digestion in a diluted acid, thoroughly washed, and 

 rapidly dried. An examination of the powder, grain by grain, 

 under a loup, still better on the stage of a microscope under a 

 low power objective, should then follow, to ensure the absence 

 of adhering or enclosed impurities and to determine with cer- 

 tainty the true color, fracture, etc. 



The determination of the crijstalline form is first necessary. 

 Even though this should be identified as orthorhombic, that of 

 marcasite, it may not be conclusive as to instability, since we 

 have seen that stable pyritic forms of this mineral exist, proba- 

 bly comprising most of the varieties, in the Marcasite series, of 

 Sp. Gr. 4.98 to 4.88, representing a content of nearly 100 down 

 to 40 per cent, of pyrite. However, the burden of evidence tends 



* Geological Sketches (1882), 178. 



