BUILDING AND ORNAMENTAL STONES. 307 



color denotes the absence of iron in any of its forms. On the condition 

 of the iron is dependent also the permanency of color. Either the sul- 

 phide, carbonate or other protoxide compounds, are liable to oxidation, 

 and hence stones containing it in these forms fade or turn yellowish and 

 stain on exposure. The sesquioxide on the other hand can undergo no 

 further oxidation, and hence the color caused by it is the most durable. 

 Hence, as a rule, the decidedly red colors may be considered most per- 

 manent. 



The blue and black colors of marbles and limestones are due largely 

 to carbonaceous matter. 



The effects of the various mineral constituents in varying the shades 

 of colors are mentioned in the chapter on rock-forming minerals and in 

 the descriptions of the different kiuds of stones. Great care and judg- 

 ment is needed in the selection of proper colors in building. Heavy 

 rock-faced walls of dull brown sandstone, dark gneiss, or diabase al- 

 ways impart an appearance of gloom, while warm, bright colors are 

 cheering aud pleasing to the eye. The late Architect Eichardson, with- 

 out doubt, owed a considerable share of his success to his power of se- 

 lecting for any particular piece of work stone of such color as to be 

 most effective and harmonious in the finished structure. 



(6) THE CHEMICAL CHARACTERS OF ROCKS. 



This naturally varies with the mineral composition and their ever- 

 varying proportions. Nevertheless, it is possible to obtain general aver- 

 ages from which the stones of each particular kind will not be found to 

 vary widety. It is customary to consider rocks which, like granite, are 

 rich in silica as acidic, while those in which, as in basalt, the average 

 percentage falls below fifty are Called basic. Various descriptive ad- 

 jectives are applied to the names of rocks according as they vary in 

 composition. Calcareous rocks consist principally of lime, or contain 

 an appreciable amount; argillaceous contain clay, which can usually be 

 recognized by its odor when breathed upon ; siliceous contain some 

 form of silica; ferruginous, iron in the form of oxide; carbonaceous, more 

 or less carbon ; bituminous contain bitumen, which can often be detected 

 by the odor of petroleum given off when the rock is freshly broken. 

 Calcareous rocks can always be detected from their effervescing when 

 treated with a dilute acid. The chemical composition of a stone is often 

 a guide to its suitability for structural purposes. Those containing 

 much lime are more liable to be unfavorably affected by the acid gases 

 of cities, and the various forms of iron present are of importance both 

 regarding the weathering properties of the stones and their colors, as 

 will be noticed later under special cases. A table of rock compositions 

 is to be found near the close of this volume. 



