^20 Decojnposition of Iron Pyrites. 



to the diabase from Somerville, Mass., in tombstones at Mt. 

 Auburn, in Cambridge, Mass. : 



** The diabase tombs have turned a rusty brown, the change 

 apparently occurring in the black minerals (augite, mica, etc.), 

 while the feldspar whitens. It is very noticeable here that 

 grains of pyrite in the stone are generally bright, without patches 

 of rust about them." Other examples of a similar resistance to 

 decomposition, by varieties of each of the three iron-pyrites, 

 have been already mentioned. In pyritiferous building-stones 

 therefore, as in pyritic coal, there now appears to be no difficulty 

 in certainly determining beforehand the character of the con- 

 tained pyrites, as to stability, by an examination of its physical 

 properties, especially color, fracture, and density, and by ex- 

 perimental trial. 



C. Pykites liN^ Roofij^g-Slate. The recognition of stabil- 

 ity in certain pyrites has not been confined to scientific students ; 

 practical men in other fields of observation are familiar with 

 them. Thus the quarrymen in the roofing-slate districts 

 of Vermont and of Wales, and roofers who make use of slates 

 from various regions, are well aware that the grains and crystals 

 of iron pyrites of particular quarries are certain to decompose 

 rapidly and discolor or disintegrate the containing slates, and 

 that such slates must be scrupulously rejected. 



It is equally recognized that, in other slates, the pyrite ap- 

 pears indestructible by the weather, and after long exposure up- 

 on roofs, remains unattacked, in brilliant hard crystals, with 

 sharp angles, and surfaces untouched by rust. I find this fact 

 also expressed in the following statements of A. Geikie ; '^Pyrite 

 when free from marcasite yields but slowly to weathering. 

 Hence its cubical crystals may be seen projecting, still fresh, 

 from slates which have been exposed to the atmosphere for 

 several generations."^ *'As a contrast to the universal decay of 

 the marble tombstones, reference may be made to the remarkable 

 durability of the clay slate which has been employed for monu- 

 mental purposes in Aberdeenshire. It is a fine grained rather 

 soft rock, containing scattered cubes of pyrites, and capable of 

 being readily dressed into thin smooth slabs. A tombstone of 



^ Text Book of Geology, (1882), 85. 



