. Discovery. Science. DISCOVERY 173 crushed, and compressed materials as foundations, with, on top, relatively thin layers of flat-lying sedi- ments. They form the permanent cores of the con- tinental masses. Compared with them the plastic zones are, in their extended form, 3,000 km. (1,800-1,900 miles) broad, shallow depressions (geosjmclines) filled with sea. Compressed they appear as zones of " chain " moun- tains having a fairly uniform width of 1,000 km. (620 miles). The Atlantic is probably an instance of such a zone in its hollow (geosynclinal) state ; the Alpine-Himalayan mou
. Discovery. Science. DISCOVERY 173 crushed, and compressed materials as foundations, with, on top, relatively thin layers of flat-lying sedi- ments. They form the permanent cores of the con- tinental masses. Compared with them the plastic zones are, in their extended form, 3,000 km. (1,800-1,900 miles) broad, shallow depressions (geosjmclines) filled with sea. Compressed they appear as zones of " chain " moun- tains having a fairly uniform width of 1,000 km. (620 miles). The Atlantic is probably an instance of such a zone in its hollow (geosynclinal) state ; the Alpine-Himalayan mountain-band is a young example of the compressed type. These zones wind sinuously around the continental blocks and every block is completely ringed in by such zones. (See map.) Formation of Mountain Zones As the earth contracts the blocks are brought closer together. By their nature and position they have little choice of movement. At first they will draw together in one and the same horizontal plane. They are the active agents, exerting pressure. The brunt of their onset must be borne by the plastic zones. These we must think of at first as in their hollow (geosynclinal) stage, sea-filled and having floors weighted (in all probability) by vast masses of heavy igneous rocks. Also, as great masses of debris from the surrounding lands keep gravitating into them (especially around their edges) their floors keep sinking and deepening and encroaching by dragging down the continental margins. When pressure is applied this deepening process is accentuated, and it appears to go on until a depth is reached where the temperature, the plasticity of the rocks, and the thinness of the earth's crust at last permit the magmas or underlying fluid strata to force their way up. The process is then reversed : pressure continues and the floor starts to bulge and buckle. The first bulging will take place in the middle of the trough and approximately along its axis ( parallel to the sides of the
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