164 STUDIES FOR STUDENTS: 
Building at Madison, Wis., reported that the Bedford limestone, 
out of which the building is constructed, was practically free from 
ferrous iron, and that the cause of the iron staining was attribu- 
table mainly to the cement used in the back wall. This is prob- 
ably a frequent cause of discoloration, on account of which good 
stone has been condemned. A common method of preventing 
the ferrous iron in the brick or mortar of the back wall from 
coming to the surface, is to use a coat of asphalt between this 
and the stone facing. A better precaution would be to select 
lime, cement, and brick from which ferrous lime is known to be 
absent. : 
A change of color through the decomposition of iron sulphide 
and carbonate is manifest mainly among the light colored rocks. 
The blue or gray limestones and dolomites are often discolored 
-by spots or irregular efflorescent patches of calcium or magne- 
sium sulphate, which appear as a white precipitate on the surface. 
Their presence at this place is attributed to interstitial water, 
which comes to the surface bearing soluble salts of magnesium 
and calcium, mainly the former. Dark colored rocks such as 
brown sandstone do not discolor, but occasionally they take ona 
lighter tint after iong exposure to the weather. This comes 
about through the loss of iron oxide which is washed off from 
the surface by the rains. Decoloration, however, takes place so 
slowly that it is not an important consideration. 
Very often, through long exposure in the quarry a rock, such 
as the blue limestone of the Trenton formation, is partly or 
entirely altered in color to a buff. Near the surface, beds may 
be found that have been completely altered, while deeper in the 
quarry one passes from those that are partly altered to those 
that are unchanged. The alteration commences along the joints 
and gradually passes toward the center of the blocks. 
The manner in which a stone is dressed sometimes affects the 
permanency of itscolor. A rough dressed stone furnishes a mul- 
titude of places in which dust and dirt may lodge, while one which 
is smooth dressed is free from such places, For this reason there 
is less danger of the original color being obscured in a smooth 
