86 EARTH FEATURES AND THEIR MEANING 



shaped as the water is sucked down at the time of the readjustment 

 with which the play of such earthquake fountains is terminated 

 (Fig. 79). Subsequent excavations made about such craterlets 

 have shown them to have the form of a trumpet, and that in the 

 sand which so largely fills them there are generally found scales of 

 mica and such light bodies as would be picked out from the hetero- 

 geneous materials of the sand layers and carried upward in the 

 rush of water to the surface (Fig. 80). 



The earth's zones of heavy earthquake. Since earthquakes 

 give notice of a change of level of the ground, the special danger 

 zones from this source are the growing mountain systems which 

 are usually found near the borders of the sea. Such lines of moun- 

 tains are to-day rising where for long periods in the past were the 

 basins of deposition of former seas. They thus represent the 

 zones upon the earth's surface which are the most unstable 

 which in the recent period have undergone the greatest changes 

 of level. 



By far the most unstable belt upon the earth's surface is the 

 rim surrounding the Pacific Ocean, within which margin it has 

 been estimated that about 54 per cent of the recorded shocks of 

 earthquake have occurred. Next in importance for seismic in- 

 stability is the zone which borders both the Mediterranean Sea 

 and the Caribbean the American Mediterranean and is ex- 

 tended across central Asia through the Himalayas into Malaysia. 

 Both zones approximate to great circles upon the earth's surface 

 and intersect each other at an angle of about 67. It has been 

 estimated that about 95 per cent of the recorded continental earth- 

 quakes have emanated from these belts. 



The special lines of heavy shock. Within any earthquake 

 district the shocks are not felt with equal severity at all places, 

 but there are, on the contrary, definite lines which the disturbance 

 seems to search out for special damage. From their relations to 

 the relief of the land these lines would appear to be lines of fracture 

 upon the boundaries of those sections of the crust that play in- 

 dividual roles in the block adjustment which takes place. More 

 or less masked as these lines are beneath the rounded curves of 

 the landscape, they are given an altogether unenviable prominence 

 with each succeeding earthquake. At such times we may think 

 of the earth's surface as specially sensitized for laying bare its 



