The composite repeating patterns of the higher orders.—Not only do the larger joint blocks become impressed upon the earth relief as repeating diaper patterns, but larger and still larger composite units of the same type may, in favorable districts, be found to present the same characters. Attention has already been more than once directed to the fact that the more perfect and prominent fracture planes recur among the joints of any series at more or less regular intervals ([Fig. 40], [p. 57], and [Fig. 41], [p. 58]). Nowhere, perhaps, is this larger order of the repeating pattern more perfectly exemplified than in some recent deposits in the Syrian desert ([plate 10 B]). It is usually, however, in the older sediments that such structures may be recognized; as, for example, in the squared towers and buttresses of the Tyrolean Dolomites ([Fig. 244]). Here the larger blocks appear in the thick bedded lower formation, the dolomite, divided into subordinate sections of large dimensions; but in the overlying formations in blocks of relatively small size, yet with similarly perfect subequal spacing.
Fig. 244.—Squared mountain masses which reveal a distribution of the joints in block patterns of different orders of magnitude. The Pordoi range of the Sella group of the Dolomites, seen from the Cima di Rossi (after Mojsisovics).
The observing traveler who is privileged to make the journey by steamer, threading its course in and out among the many islands and skerries of the Norwegian coast, will hardly fail to be struck by the remarkable profiles of most of the lower islands ([Fig. 245]). These profiles are generally convexly scalloped with a noteworthy regularity, and not in one unit only, but in at least two with one a multiple of the other ([Fig. 246]). As the steamer passes near to the islands, it is discovered that the smaller recognizable units in the island profiles are separated by widely gaping joints which do not, however, belong to the unit series, but to a larger composite group ([Fig. 246 b]). Frostwork, which depends for its action upon open spaces within the rocks, has here been the cause of the excessive weathering above the more widely gaping joints.
Plate 10.
A. View in Spitzbergen to illustrate the disintegration of rock under the control of joints. (Photograph by O. Haldin.)
B. Composite pattern of the joint structures within recent alluvial deposits. (Photograph by Ellsworth Huntington.)