CHAPTER VI
THE ARCHITECTURE OF THE FRACTURED SUPERSTRUCTURE
Fig. 36.—A set of master joints developed in shale upon the shores of Cayuga Lake near Ithaca, New York (after U. S. G. S.).
Fig. 37.—Diagram to show how sets of master joints differing in direction by half a right angle may abruptly replace each other.
Fig. 38.—Diagram to show the different combinations of the series composing two double sets of master joints, and in a, a, a additional disorderly fractures.
The system of the fractures.—In referring to experiments made upon the fracture of solid blocks under compression ([p. 41]), it was shown that two series of parallel fractures develop perpendicular to each free surface of the block, and that these series are each of them inclined by half of a right angle to the direction of compression, and thus perpendicular to each other. The fragments into which a block with one free surface would thus tend to be divided should be square prisms perpendicular to the free surface. It would be interesting, if it were practicable, to learn from experiment how these prisms would be further fractured by a continuation of the compression. From mechanical considerations involving the resolution of forces with reference to the ready-formed fractures, it seems probable that the next series of fractures to form would bisect the angles of the first double series or set. Wherever rocks are found exposed in their original attitudes, they are, in fact, seen to be intersected by two parallel series of fractures which are perpendicular to the earth’s surface and to each other and are described as joints. In many cases more than two series of such fractures are found, yet even in these cases two more perfectly developed series are prominent and almost exactly perpendicular to each other as well as to the earth’s surface. This omnipresent double series or set of joints is the well-known set of master joints, and very often it is found developed practically alone ([Fig. 36]). Over large areas, the direction of the set of master joints may remain practically constant, or this set may quite suddenly give place to a similar set which is, however, turned through half a right angle from the first ([Fig. 37]). Not infrequently two such sets of master joints are found together bisecting each other’s angles within the same rocks, and to them are sometimes added additional though less perfect series of joint planes.