2. The mountain-forming sediments are usually strongly folded and faulted, and exhibit slaty cleavage wherever they are susceptible of that structure; and the older rocks, especially, in mountains are often highly metamorphosed, and are traversed by numerous veins and dikes, the infallible signs of intense igneous activity. “In other words, mountain regions have been the great theatres—(1) of sedimentation before the mountains were formed; (2) of plication and upheaval in the formation of the range; and (3) of erosion which determined the present outline. Add to these the metamorphism, the faults, veins, dikes, and volcanic outbursts, and it is seen that all geological agencies concentrate there.”—LE CONTE. Since mountain-ranges are great up-swellings or bulgings of the strata, their structure is always essentially anticlinal; and they sometimes consist of a single more or less denuded anticline (Fig. 38), the oldest and lowest strata exposed forming the summit of the range. More commonly, however, the single great arch or uplift is modified by a series of longitudinal folds, as shown in the section of the Jura Mountains (Fig. 21). Still more commonly the folds are closely pressed together, overturned, broken, and almost inextricably complicated by smaller folds, contortions, and slips. [Illustration: Fig. 38.—Anticlinal mountain.] The strata on the flanks of the mountains are usually less disturbed than those near the axis of the range, and are sometimes seen to rest unconformably against the latter. In this way it is proved that some ranges are formed by successive upheavals. But we have still more conclusive evidence that mountains are formed with extreme slowness in the fact that rivers sometimes cut directly through important ranges. This proves, first, that the river is older than the mountains; second, that the deepening of its channel has always kept pace with the elevation of the range. CONCRETIONS AND CONCRETIONARY STRUCTURE.—Folds, cleavage, faults, and joints—all the subsequent structures considered up to this point—are the product of mechanical forces. Chemical agencies, although very efficient in altering the composition and texture of rocks, are almost powerless as regards the development of rock-structures; and the only important structure having a chemical origin is that named above.