170 STUDIES FOR STUDENTS 
of hardness may have a comparatively low crushing strength and 
elasticity. Any increase in the percentage of this material will 
increase the hardness of the rock at the expense of strength and 
elasticity. Of course, the elasticity, hardness, and strength are- 
not controlled by the one factor of abundance. A rock may 
consist entirely of the strongest minerals, and yet the size, man- 
ner of contact, and arrangement may be such that it will be one 
of the weakest. 
TEXTURE OR STATE OF AGGREGATION 
As outlined above the texture of a rock has reference to the 
size, shape, manner of contact, and arrangement of the mineral 
particles. The size of the particles affect the weathering of a 
stone by increasing the differential disintegration. When the 
mineral particles are large they disintegrate and weather out 
‘most easily, often leaving small depressions, on account of which 
the surface has a pitted appearance. The larger mineral par- 
ticles have more pronounced cleavage cracks which increase the 
rate of weathering. Chemical agents have a better chance to 
operate and the stone is weakened throughout. Rocks which 
are composed of small mineral particles may have correspond- 
ingly small pore spaces, although the size of the pores is largely 
controlled by the shape and manner of contact of the grains. 
The shape and manner of contact of the grains influence 
the strength and durability of the stone, as much perhaps as any 
of its other qualities. If the grains are close fitting the adhesion 
will be increased and the pore space decreased. When the 
grains are irregular in outline they usually interlock after the 
manner of dovetail work, which adds to the strength and lessens 
the pore space. 
Upon the arrangement of the grains depends the laminated 
schistose, or cleavage structure in rocks. Ifthe mica or other 
minerals are arranged with their longest axes in a common direc- 
tion and concentrated along certain planes the rock will possess 
a capacity to part most readily in that direction and along those 
planes. The perfection of development of the parting capacity 
will be influenced also by the size of the grains. 
