300 KEPORT ON NATIONAL MUSEUM, 1886. 



viridis, green, this being its usual color. The characteristic greenness 

 which gave the name greenstone to tlie diorites and diabases is due, in 

 large part to the secondary chlorite contained by them. 



IRON PYRITES. — Composition : Iron disulpbide, FeSj = sulphur, 53.3 per ceut.; 

 iron, 46.7 per cent. Hardness, 6 to 6.5. 



A very common accessory in rocks of all kinds and all ages, usually 

 occurring in small cubes or irregular masses of a brassy yellow color. 



It may be set down as a rule that rocks containing this mineral should 

 not be used for ornamental work that is to be exposed to the weather, 

 siuc<i it is very liable to oxidation in time, staining the stone and per- 

 haps causing llic more sei ions result of disintegration. This form of 

 the iron disnlpliide is, however, less objectionable than that known as 

 marrasite or the gray iron pyrites. 



For some unexi^lained reason this form of the mineral decomposes 

 even more readily than the p^'rite, and hence its presence is always to 

 be avoided in all rocks where permanency of color or durability is de- 

 sired. 



A microscopic study of pyritebearing rocks has shown that there 

 are many important considerations bearing ui^ou the weathering prop, 

 erties of this mineral. Thus it is found, as in many of the Ohio lime- 

 stones and dolomites, occuring not onl^MU well-detined cubes of a brassy 

 yellow color, but also in an amorphous granular condition in a ver^- 

 fine state of subdivision which appears almost black under the micro- 

 scope. Experience has shown that in the latter form it is much more 

 liable to oxidation than when in cubes, and hence we see the necessity 

 of a microscopic examination of a stone as one of the guides to its i)rob- 

 able weathering qualities. In this finely amorphous condition the pyrite 

 is stated by Ilawes to have an important effect upon the color of the 

 stone. Thus the Springfield and Covington (Ohio) dolomites present 

 in different layers two well defined colors — a blue and a yellow. An 

 examination with the microscope shows that they differ only in that 

 tlie blue variety contains the pyrite in the finely disseminated uuoxi- 

 dized state, while in the yellow it has become changed into the hydrous 

 oxide. This change having taken place while the stone lies in the 

 quarry, is unaccompanied by results of a serious nature, unless the uni- 

 form change in color be so considered. Had the change taken x^lace in 

 the quarried stone after being laid in the walls of a building, the results 

 would 111 all probability have proved more undesirable. Pyrite when 

 imbedded firmly in rocks of a close, compact nature is less liable to oxi- 

 dation than when contained in one of a loose and porous texture. In 

 the magnesian limestones of Dayton, Ohio, the microscope reveals many 

 minute cubes of pyrite which are imbedded so firmly in its mass as to 

 be not at all deleterious, since beyond the reach of atmospheric agencies. 

 In many close-textured rocks, as the slates, pyrite is proverbially long- 

 lived, and hence as a rule we can only regard it with suspicion, as an 



