ON SEISMOLOGICAL INVESTIGATION. 63 



the north-south ridge the slopes to the west over a short distance like 

 twelve miles is 1,000 feet per mile, whilst to the eastwards it is compara- 

 tively gentle. If these gradients be measured in lengths of 200 miles the 

 slopes are about 70 feet per mile. 



D. Andean District (number of earthquakes 12). — At many points on 

 the west coast of South America, within fifty miles of the shore, deptlis of 

 from 2,000 to 4,000 fathoms occur, which correspond to gradients of from 

 250 to 480 feet per mile. Within a distance of 150 miles from the shore 

 the land rises to a height of 12,000 feet, so that the gradients from them 

 to the bottom of the neighbouring ocean may be taken at 120 to 180 feet 

 per mile. 



E. Japan District (number of earthquakes 29). — To the east of 

 Northern Japan and the Kuriles, at a distance of about 180 miles off shore, 

 depths of 4,000 and even 4,600 fathoms are found, indicating gradients of 

 130 to 150 feet per mile, and from observations made in Japan it is known 

 that many of the large earthquakes originate on the face or at the bottom 

 of these slopes. 



F. Javan District (number of earthquakes 41). — Ofi' the south-west coast 

 of Sumatra and the south coast of Java, at distances of from eighty to 100 

 miles, depths of from 2,000 to 3,000 fathoms occur. The straits on the 

 opposite shores of these islands are shallow, seldom exceeding thirty 

 fathoms. Eastwards, from Java as far as Ceram, soundings between 

 1,000 and 2,000 fathoms are frequent. At one point tifty miles south 

 of the latter island there is a depth of 4,000 fathoms. From this 

 particular 'deep' on September 29, 1899, a displacement took place the 

 effects of which were partially visible by subsidences on the southern coast 

 of Ceram. In this district the sub-oceanic irregularities in contour are 

 as irregularly distributed as the islands which form their outcrops. 



G. Mauritian District (number of earthquakes 17). — The origins for this 

 group of earthquakes are not well defined. They are probably related to 

 the depression lying between the ridges represented by the Laccadi^'es 

 and Maldives on the east, and the Seychelles and Mascarine Islands to 

 the south-west. 



H. North-eastern Atlantic (number of earthquakes 22). 



I. North-western Atlantic (number of earthquakes 3). 



J. North Atlantic (number of earthquakes 3). 



The earthquakes originating in these districts have been few in number, 

 comparatively small, and their origins are not well defined. Although a 

 ridge is marked as extending up the Atlantic, it is compai'atively small, 

 and even in the vicinity of the Azores it is difficult to find a gradient over 

 a distance of 180 miles which exceeds 33 feet per mile. 



K. Alpine, Balkan, Caucasian, Himalayan Districts (number of earth- 

 quakes 14). — Strictly speaking this region, which is the only one from 

 which earthquakes originate on a land surface, might be divided into four 

 or more sub- regions according to the direction of the strike of the ridges 

 which each I'epresents. 



The most pronounced foldings are in the eastern part of these districts 

 where in distances of 100 miles gradients of 120 feet per mile can be 

 found, and it is from these steep slopes that the larger earthquakes have 

 originated 



