GEOLOGIC FEATURES OF THE COMMON ROCKS
To describe the geologic features of the common rocks used in commerce would require a full treatise on the subject of geology. These are the bulk materials of the earth and in them we read the geologic history of the earth. In preceding chapters a brief outline has been given of the relative abundance of the common earth materials and of the processes producing them. In comparison, the metalliferous deposits are the merest incidents in the development of this great group of mineral resources.
In this section reference will be made only to a few of the rock qualities and other geologic features which require first attention in determining the availability of a common rock for commercial use. The list is very fragmentary, for the reason that the uses are so many and so varied that to describe all the geologic features which are important from the standpoint of all uses would very soon bring the discussion far beyond the confines of a book of this scope.1
Building Stone
For building stones, the principal geologic features requiring attention are structure, durability, beauty, and coloring.
The structures of a rock include jointing, sedimentary stratification, and secondary cleavage. Nearly all rocks are jointed. The joints may be open and conspicuous, or closed and almost imperceptible. The closed joints or incipient joints cause planes of weakness, known variously as rift, grain, etc., which largely determine the shapes of the blocks which may be extracted from a quarry. Where properly distributed, they may facilitate the quarrying of the stone. In other cases they may be injurious, in that they limit the size of the blocks which can be extracted and afford channels for weathering agents. Some rocks of otherwise good qualities are so cut by joints that they are useless for anything but crushed stone. The bedding planes or stratification of sedimentary rocks exercise influences similar to joints, and like joints may be useful or disadvantageous, depending on their spacing. The secondary cleavage of some rocks, notably slates, enables them to be split into flat slabs and thus makes them useful for certain purposes.
Proper methods of extraction and use of a rock may minimize the disadvantageous effects of its structural features. The use of channelling machines instead of explosives means less shattering of the rock. By proper dressing of the surface the opening of small crevices may be avoided. Stratified rocks set on bed, so that the bedding planes are horizontal, last longer than if set on edge.
The durability of a rock may depend on its perviousness to water which may enter along planes of bedding or incipient fracture planes, or along the minute pore spaces between the mineral particles. The water may cause disastrous chemical changes in the minerals and by its freezing and thawing may cause splitting. For this reason, the less pervious rocks have in general greater durability than the more pervious. Highly pervious rocks used in a dry position or in a dry climate will last longer than elsewhere.
Durability is determined also by the different coefficients of expansion of the constituent minerals of the rock. Where the minerals are heterogeneous in this regard, differential stresses are more likely to be set up than where the minerals are homogeneous. Likewise a coarse-textured rock is in general less durable than a fine-textured one. Expansion and contraction of a stone under ordinary temperature changes, and also under fire and freezing, must necessarily be known for many kinds of construction.