330 STUDIES FOR STUDENTS 
glaciers as they passed over the region in question, and the 
length of time that the surfaces have been exposed to the atmos- 
phere since glaciation. 
The durability of a stone is occasionally estimated from the 
depth to which disintegration has extended. The extent of dis- 
integration, however, has the uncertain time element in it, which 
may vitiate the conclusions. 
Observations on stone in the natural exposure, where disinte- 
gration has not gone too far, reveal inequalities in hardness 
caused by concretions, nodules, pebbles, clay seams or pockets, 
and fossils. Weathering also emphasizes sedimentary and joint- 
ing planes, which are obscure in the freshly quarried rock. It is 
often contended by quarrymen that joints die out with depth, 
but this cannot be laid down as a general rule. Some joints are 
probably superficial, but others certainly penetrate to very con- 
siderable depths. The farther the joints extend laterally, the 
greater is the probability that they will continue to a considera- 
ble depth. 
A luxuriant growth of lichens on a natural exposure of rock 
is frequently taken as evidence of durability. Unfortunately this 
criterion of durability, when taken alone, has little significance. 
An abundant growth of lichens has often been observed on the 
surface of sandstone which was inherently soft. Such occur- 
rences simply indicated that a crust had been formed on the 
exposed surface of the stone. 
If one is desirous of obtaining a considerable quantity of stone 
perfectly uniform in color and texture, it is important that he 
should visit the quarry to assure himself that the amount of 
stone of the desired quality is obtainable. It is possible for a 
quarry to be exhausted of its good stone, and for this reason, an 
inspection is often a valuable precaution. On the other hand, 
the stone from a certain quarry, which has a large percentage of 
number one stone, may have been condemned by the public, 
because quarrymen and contractor have permitted the use ofa 
few interior blocks, “tor the sakesotteconomy a) inondenite 
know when the best stone that a quarry produces is being 
