188 REPORT— 1897. 



result in contours which have reached limits of stability ready to yield as 

 more materials accumulate, by facial slidings, by overloading, by changes 

 in currents, by seismic action, and in other ways. 



Forms of Stability. — On land we have many illustrations of natural 

 curves of stability. A volcano mainly consisting of lapilli which have 

 accumulated round a central orifice has a form dependent upon the 

 density and strength as represented by resistance to crushing of its com- 

 ponent materials. To increase the height of such a mountain, it would be 

 necessary to increase the area of its base. The upper portion of Mount 

 Fuji has a slope of 30°, but as we proceed downwards the slope becomes 

 less and less until at last it is asymptotic to the plain from which it rises. 

 'The average slope of this volcano is 15°. 



If, therefore, on the face of a bank formed by the accumulation of 

 .sediments, soundings, taken at points separated by one or more miles, 

 indicate a certain inclination, it may be inferred that the steepest slope 

 may possibly greatly exceed the quantity thus determined. 



The only experiments bearing upon slopes of stability formed beneath 

 water with which the writer is acquainted are a few made by himself. 

 These experiments, which were made with sand and carried out in various 

 manners, pointed to the following general results : — 



1. Sediments deposited under the influence of currents accumulate in 

 slightly flatter forms than those of similar materials built up on land. 



2. Peaks, edges and corners of loose materials which may be fairly 

 stable on land are beneath water, even when it is still, quite unstable, and 

 quickly become rounded. 



3. A mound or bank when thus rounded is very stable even under the 

 influence of strong currents, but the unstable form may be quickly repro- 

 duced by the accumulation of new sediments. 



The conclusions then are, first, if we find beneath water very short 

 slopes of detrital materials, if they are 2° or 3° less than the angle at 

 which similar materials ai^e self-supporting on land, they have reached a 

 limit of stability ; and, secondly, average slopes over distances of one or 

 more miles indicate the existence of much steeper slopes over shorter 

 lengths. 



Causes resulting in the Yielding of Submarine Banks. — Because it is 

 not likely that submarine earthquakes the movements of which are felt 

 round the world are the result of volcanic action whenever these are 

 accompanied by sea-waves, it may be inferred that the latter have been 

 produced by the di.sloclgment of vast masses of material from the faces of 

 steep slopes. Illustrations of such changes will be given in the next 

 section. 



That intermittent facial sliding takes place on steep slopes during the 

 accumulation of new materials is rendered likely by what we observe 

 taking place on the faces of a mound of sand, submerged beneath water, as 

 it grows upwards as an accumulation from a fine stream of sand descending 

 from above. 



Basal crush with horizontal displacement would only be expected to 

 occur around the lower edges of slopes of great height ; and as it is hardly 

 reasonable to suppose that such slopes owe their form simply to the 

 accumulation of sedimentary deposits, then the frequent origin of 

 earthquakes in such localities indicates that the pi'imary cause of crush or 

 thrust is the result of yielding in rocky masses rather than that of 

 detritus. When speaking of cable-interruptions it will be seen that some 



