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 (i.e. parallel 

 to the sides of the in pressing blocks). The seas will 

 now begin to be forced out and their encroachment on 



Saa Level 



Fig. 2.— the " PRESSING OUT " OF AX OROGENE TROUGH. 



the lands (see above) will be accentuated. As pressure 

 continues other ridges form on the floors of the trough, 

 parallel to the original bulge and extending outwards 

 on either side from it. If there are any harder lumps 

 in the floor they will serve first as occasions and then 



as the cores of the growing folds (as a dead camel may 

 cause a sand-hill). The blocks press in, the ridges 

 press outwards. Pressure intensifies : the ridges 

 jostle, mount up, push over one another, press some 

 up and out, others in and down, in the search for room 

 and still more room. The deeper and more compressed 



Orogene^ 

 Zone ! 



Fig. 3.— diagram to show movements IN" OROGENE ZONE. 



parts are more plastic and serve as gliding planes for 

 the more refractory materials above. The trough is 

 now truly an orogenic zone, a zone giving birth to 

 mountains. Ridge mounts on ridge, and above the 

 shallowing out-flooding seas rise peaks, island rows, 

 island chains, archipelagoes. Then up the inclined 

 planes formed by the continental margins on either 

 side press the ridges, layer thrusting over layer, 

 advancmg against the solid land like great frozen 

 waves, their steepest " breaking " faces always in 

 front (outwards). Last of all the whole system is 

 pressed up high and dry : the geosyncline is " pressed 

 out " ; it is one-third of its original breadth ; it has 

 become a zone of mountains. (Figs. 2-6.) 



These mountain zones are eloquent of their origin. 

 Their materials reveal their birthplaces — the shallow, 

 deep, or abysmal seas or the deeper rock zones under- 

 lying these. Their structure, infinitely complicated 

 by reason of their history, can generally be imravelled, 

 and this structure is most striking. 



Every orogene zone is two-faced. It is as though 

 two giant armies had advanced, back to back, each 

 against an onpressing continent, and each had been 

 frozen where it stood. In the front line are the tallest 

 aggressors frowning abruptly out upon the lands. 

 Where thej'tread the plains subside, and the Himalayas, 

 the chains of Iran (and many others) are fronted by 

 deeps (Ganges Valley, Persian Gulf, Mesopotamia). 

 The line of advance may be in any direction of the 

 compass, but always it is agamst the continental 

 block. (See Fig. 3.) Sometimes the advancing giants 

 shovel or push up the land in front of them, and then 

 they are confronted by a shelf-like plateau (Colorado 

 Plateau). (See Fig. 6.) Where the great blocks 

 press hard together, there is intensest distortion and 

 a knot (Pamirs ; Armenian Knot) ; where the pressure 

 is less intense, there are interment areas, generally 

 plateaux (interior Asia Minor ; Thibet). These, 

 having been forced up from the trough floors, are 



