86 THE FLOOR OF THE OCEAN 



crust was bent down, displacing the material of the third, 

 weak layer; and that the top, sialic, layer was, as it were, 

 scraped off the rest of the crust and accumulated as a crumpled 

 and sliced, over thrust mass under the region marked "Strip." 

 Toward its bottom the sial has attained a so-called "root." A 

 root of just this kind has been demonstrated under the Swiss 

 Alps, where, in an analogous way, the superficial rock forma- 

 tions were set writhing and piled on one another when Europe 

 and Africa were pushed together. In both belts the extra 

 thickness of the relatively light sial gives buoyant support to 

 the deformed masses. 



While the Vening Meinesz theory is the best in sight, it is 

 well to note that to a considerable extent it is founded on in- 

 direct evidence. Where the thing to be explained is so largely 

 veiled by the ocean, it is well to consider the grounds for 

 approval of the theory. We are to find that the evidence is in 

 part geological and in other part geographical, and that much 

 of the reasoning is from the analogy of the "negative strip" with 

 the visible, accessible mountain chains of the continents. For 

 reasons more or less obvious, the analogy is not perfect, but in 

 this very imperfection we shall find food for thought about the 

 mountain-making process — one of the most baffling problems 

 of earth science. 



Let us run through a list of characteristics for the visible 

 cordilleras of the world. Detailed study of each of the moun- 

 tain-structures shows it to be the product of localized folding, 

 crushing, and slicing of sialic rock, the deforming pressure 

 having been essentially horizontal. The folding is illustrated 

 in Figure 49. The series of vertical cross-sections of Figure 50 

 may serve to show how, under intense and continuing horizon- 

 tal pressure, thick sheets of rock of the sialic or continental 

 type are sliced apart and driven over one another. 



