174 



DISCOVERY 



mostly of heavier materials, and, when the pressure is 

 relaxed, they may subside and form basins (Hungarian 

 Plain). ^ Where the advance is unopposed, it is in 

 line (Pyrenees, Caucasus) ; where it is held up at 

 points, the rest of the front moves forward into great 



betray the stresses which the continental block has 

 suffered. (See Asia on map, Fig. i.) Sometimes even 

 a whole block seems to be depressed or elevated : in 

 the Pacific a block (or two blocks), after having helped 

 to create the great girdle chains, seems to have simk. 



riGS. 4 .«VD 5.— KOBER'S DI.iGRAMS ILLUSTRATING THE PRESSING UP OF THE OROGENE TROUGH INTO ISLANDS AND ISLAND CHAINS. 



arcs [Aleutian, Japanese (East Asian), Carpathian, 

 Dinaric, Tauric arcs]. 



Concretion of Continental Masses 



Though stiff and unyielding, the Old Blocks do not 

 come off unscathed. Comparatively free from the 

 volcanic and seismic travail which marks the birth 

 and death of mountains, they yet buckle, crack, tilt, 

 and sag under the tremendous strain. Every block is 

 ringed round by an orogenic zone, and exerts and 

 suffers pressure from all sides. Doubtless the incidence 

 of the pressure is unequal both in place and time, and 

 mountain zones — or even single mountain chains — 

 must not be thought of as rising or sinking simultane- 

 ously and evenly always and all over the globe. Never- 

 theless, the great tectonic disturbance lines of the 

 continents — the lines of new, old, rejuvenated moun- 

 tains, of scarps and fracture-valleys — are ranged 

 concentrically around the continental blocks, the most 

 violent around the edges and fading away gradually, 

 like ripples, inwards so that the hearts of the conti- 

 nents remain almost unmovtd. Such dominant lines, 

 by their insistence and their compromises, clearly 



Fig. 6.— KOBER'S diagram SHOWING (i) THE FORMATION OP 



A TYPICAL OROGENTJ MOUNTAIN SYSTEM; (2) THE PUSHING 



UP OF THE EDGE OF THE CONTINENTAL BLOCK UNDER 



PRESSURE (COLORADO PLATEAU). 



' Kober points out that in the great Mid-World (Mediter- 

 ranean) Orogene System these intermont blocks sink steadily, 

 along with the sinking of the whole mountain system, as we 

 go westwards from its highest part in Central Asia. Thus 

 Thibetan Plateau: average elevation c. 12,000 ft. (cf. 

 Roof of the World); Persian Plateau : c. 6,000 ft. ; Asia Minor 

 Plateau; c. 3,000 ft.; Hungarian Plain: c. 300 ft.; West 

 Mediterranean Basin : below sea level. 



Still, in spite of temporary flooding and submerg- 

 ences, whole or partial, the blocks are permanent 

 elements in' the earth's crust. They are continental 

 cores, units from which continents are built. For the 

 last fate of a geosyncline is to be permanently " pressed 

 out," " landed," to become a rigid mountain zone 

 soldering two blocks together. Thus Eurasia is 

 compacted of at least three blocks permanently con- 

 creted and, in spite of the superficial flooding of the 

 Mediterranean area, it is likely that Africa also is 

 now finally cemented to Europe. 



Subsidence of Mountain Systems 



But this happens seldom. The materials squeezed 

 up from the geosyncline are relatively loose, and they 

 are forced up to immense heights. They therefore 

 tend to settle. Also, though for the most part rela- 

 tively light, they represent in the mass an enormous 

 weight. They are pUed up on the continental margins, 

 and these, unequal to the strain, sag. The whole 

 orogene structure is, in fact, unstable, as the vast 

 ruins of water-logged mountain systems, especially 

 along the continental margins of the world, attest. 



Thus it comes that the seas, which were sent flooding 

 out far and wide by the steady rise of the mountain 

 zone, contract again and drain back as the continental 

 edges — and, indeed, the whole orogene system — sub- 

 side. This subsidence is due ultimately to continuous 

 earth contraction. We witness the rebirth of an ocean. 

 Spent, as it were, by its mighty effort, the mountain 

 zone sinks back and in its place is a trough of the sea. 

 Along with it it has carried (to various depths) large 

 portions of the surrounding continents, and the existing 

 land-masses are little more than cores, stiff kernels, 

 of much larger structural units. [See Fig. i (map)]. 

 On the floor of the new oceanic geosyncline are preserved 

 at least the vague lineaments of the orogenic zone, its 

 1,000 km. broad highland, its great frontal deeps, and 

 in addition the great flanking hollows caused by 

 continental subsidence. (See Atlantic on map.) 



