TEE ROPER IES OF RULEDING SLONES- ELG. 335 
Stone from different parts of the same quarry may differ 
widely in mineralogical composition, strength, hardness, elas- 
ticity, and porosity. Differences in these respects, when of 
importance, may be detected through quarry observations. How- 
ever, in order to ascertain these differences, one needs to be 
thoroughly familiar with the conditions controlling these prop- 
erties. Differences in mineralogical composition may be recog- 
nized by one who is familiar with the common rock-forming 
minerals. 
Differences in the strength of stone result from differences 
in mineralogical composition, and in size, shape, and manner of 
contact of the individual grains, all of which can be made out by 
an experienced observer. 
Not only can an experienced person detect differences in 
the qualities of stone from various parts of the same quarry, but 
he can also make comparisons with stone from other known 
localities. 
At the quarry the unweathered stone can be compared with 
that of the natural outcrop which has been exposed to the 
atmosphere for many years. Comparisons based upon such 
observations furnish very fair estimates of the permanence of 
color and the degree of hardness, strength, and durability of the 
stone. Such estimates, however, must necessarily be very gen- 
eral, because of the uncertain length of time that the weathered 
stone has been exposed to the atmosphere. It may have been 
uncovered for centuries, or perhaps for only a few years. 
In the outcrop the stone may be found bleached, stained with 
brown, or discolored with white efflorescent patches. Bleaching 
ordinarily proceeds very slowly and extends to no great depth, 
and is therefore of little importance. Brown staining usually 
indicates the presence of iron, and the white efflorescent patches 
give evidence of magnesium or calcium salts. 
In glaciated regions the hardness of a rock is frequently esti- 
mated by the depth and extent to which the surface has been 
grooved or striated. However, this is a very uncertain evidence 
of hardness, being controlled largely by the condition of the 
