180 STUDIES FOR STUDENTS 
particles, separate them from one another, and carry them 
away. These particles may, in turn, as they are carried down 
the side of the building, wear off other particles, and so on 
until the bottom is reached. The effects of drifting sand, 
that are such conspicuous features of the arid regions, are very 
slight in the temperate zone in which we live. Drifting sand 
contributes an almost insignificant part to the whole process of 
disintegration. "J. C. Smock, in his report on the building stone 
of New York, mentions the fact that the ground glass character 
of many of the window panes in some of the older houses of 
Nantucket are due to driven sand. The windward sides of many 
of the monuments in the older eastern cemeteries have lost their 
polish, while in some cases even the lettering has been destroyed 
by this same agent. The monuments in the cemeteries of Wis- 
consin which are located in sandy regions are beginning to show 
the effects of wind-blown sand. The polish is dulled and the 
lettering is becoming indistinct. 
Besides being subject to the action of wind-blown sand and 
rain, stone is often used in places where it is abraded by thou- 
sands of feet passing over its surface. There is a great differ- 
ence in the capacity which different stones possess to withstand 
abrasion. Sidewalks, pavements, and steps may be seen in every 
city which are more or less worn by constant shuffling of feet 
over their surfaces. 
Growing organisms.—It is a very common occurrence to find 
lichens and alge covering the surface of a rock in a quarry. 
Trees may also be observed sending their roots deep into the 
crevices and cracks of the rock, and by their growth and expan- 
sion huge blocks are often broken from the parent mass. In 
some of the very soft rocks the writer has observed the finer 
rootlets ramifying through the body of the rock itself, destroy- 
ing the adhesion which bound the particles together. Decaying 
plants are also known to give off organic acids which aid in the 
decomposition of the rock. Fungi and alge often attach them- 
selves to the stone, frequently almost entirely covering the 
exposed surface. The most common form of plant growth 
