338 REPORT ON NATIONAL MUSEUM, 1886. 



Iron, in the form of magnetite — a mixture of the ferrous and ferric 

 oxides — is liable to still farther oxidation, becoming converted wholly 

 into the hydrous or anhydrous ferric oxide. Thus, if abundant, the 

 rock assumes a rusty hue, and perhaps gradually falls away to a coarse 

 sand, as is the case with certain of our diabases.* 



Black mica, hornblende, augite, and other silicate minerals rich in iron 

 are also liable on long exposure to change through the further oxidation 

 of this ingredient, but when a stone is placed high and dry, as in the 

 walls of a building, this change must necessarily be so slow as to be of lit- 

 tle moment, though of the greatest importance from a geological stand- 

 point. Mr. Wolff, however, states t that tombstones of diabase in ceme- 

 teries about Boston have in some cases turned a rust-brown color, the 

 change apparently occurring in thehornblendeandaugite. Thefeldspars 

 of the granites used in this same city were also observed in many cases 

 to have become liver-brown, rusty-red, or yellow owing to the higher 

 oxidation of the iron contained by them. 



Deoxidation. — The process of deoxidation, whereby a ferric is chauged 

 to a ferrous oxide, is possible generally only in presence of organic 

 acids and continual moisture. It is likely, therefore, to affect only those 

 stones used for foundations, and need not be further considered here. 

 The same may be said in regard to hydration, whereby an anhydrous 

 is changed to a hydrous oxide. The blotching and variegation of beds 

 of sandstone, as those of Marquette, Mich., is due to the deoxidation 

 and hydration of the iron oxides forming their cement, together with a 

 partial removal of the same by the aid of organic acids. Such changes 

 are presumably possible only in the quarry bed or in moist foundations 

 and bridge abutments. 



Solution. — The subject of solution can not, however, be passed over 

 so lightly. Pure water alone is practically without effect on all stones 

 used for building purposes. Rain-water, however, as already noted, 

 may contain appreciable quantities of various acids which greatly add 

 to its solvent power, as the rapid destruction of certain classes of 

 rocks only too well attests. Carbonate of lime, the material of ordi- 

 nary marbles and limestones, is particularly susceptible to the solvent 

 action of these acids even when they are present in extremely minute 

 quantities, and to this agent is largely due the rapid defacement of the 

 marble tombstones in church-yards and the marble faced buildings in 

 cities. 



It is to the ready solubility of calcium carbonate that is due in large 

 part the poor weathering qualities of sandstones with calcareous cem- 

 euts. The calcite is slowly removed by solution ; the silicious grains thus 

 become loosened, and, falling away under the influence of wind and rain, 



* Id one part of the dikes that form the Hanging Hills at Meriden, Conn., the rock 

 (diabase) is quite hlaek, and the amount of iron (nearly 14 per cent, of magnetite) 

 has been the cause of rapid disintegration. Hawes. Am. Jour. Sci., Vol. ix, od, 



1875, p. 188. 

 t Rep. Tenth Census. 



