3S8 BULLETIN OF THE NEW YORK STATE MUSEUM 



3. The hydration produced in the case of the iron oxides 

 and anhydrous minerals. 



4. The action of sulphurous acids in the air. 



The action of carbonated waters, in dissolving mineral 

 matters of rocks, is one of the best known facts in chemical 

 geology. And the extent to which some limestones, and 

 even crystalline rocks are weathered, bears witness to its en- 

 ergy. In the laboratory it is possible to imitate nature by 

 subjecting specimens to this test. Carbonate of lime and 

 oxide of iron, which are often the cementing matter in a 

 stone, are particularly susceptible of solution by rain-water 

 containing some acid.'^' 



The kaolinization of the feldspars is another form in 

 which the solvent action of water is exhibited. This phase 

 of decomposition is common in nature, and the kaolin beds 

 are results of this decay. More observations on its occur- 

 rence in our granite structures are wanting, owing to the 

 short periods which they have stood exposed in our climate. 



Careful microscopic examinations would doubtless prove 

 its progress in existing structures. 



The oxidation of pyrite and magnetite, and of the ferrous 

 compounds of iron, is often attended with serious results, 

 and nearly always produces discoloration and makes a stone 

 unsightly.f In the case of magnetite, the oxidation pro- 

 cess results in the production of a ferric oxide, and yellow- 

 ish or red stains and spots which disfigure the stone. As 

 in the case of the pyrite, there appears to be some varia- 

 tion in the tendency to oxidation, and the presence of mag- 

 netite is not always absolutely injurious or liable to cause 

 decay and mar the beauty of the stone. Examples of 

 stained and spotted stone, due to oxide of iron, are so com- 



*The tests referred to on pages 367, 368 show the extent to which the several classes 

 of building stone are affected, when subjected to the action of carbonic acid gas. 



f When the pyrite is in fine crystals and distributed uniformly, the oxidation may 

 produce a mellowing tint and pleasing effect, as in some of the Ohio sandstones. — 

 Merrill, Building Stone, p. 337. 



