THE STRENGTH OF THE EARTH'S CRUST 



43 



subtracting the weight of the water from that of the sediments. 

 A specific gavity of 2.67 has been taken by geodesists as the aver- 

 age for the outer shell of the earth. The degree of consolidation 

 of the deeper parts of the deltas is not known, but for present pur- 

 poses the specific gravity of their sediments as a whole may be 

 assumed as 2.50. This will be near the truth if the composition 

 is that of the average shale, if 10 per cent of pore space be assumed 

 and this is wholly filled with water. The specific gravity of sea 

 water is i . 03 , leaving an effective specific gravity for the sediments 

 of 1.47. The ratio of 1.47 to 2.67 is 0.55. The thicknesses 

 given for the deltas should therefore be multipled by this factor for 

 estimating the equivalent burdens of rock of specific gravity of 

 2.67 above sea-level. 



400 km. 



=__J?SeQ level 



1000 m. 



200O 



3000 

 tooo 



Fig. 4.— Vertical section of the delta of the Niger on A- A, Fig. 3. Horizontal 

 scale 1 : 5,000,000. Vertical scale i : 200,000. Area of the section, 645 kilometers. 



It is seen that the deltas are in the form of inclined double 

 convex lenses. Thicknesses approaching the maximum are found 

 over considerable areas in the middle. The load imposed by this 

 thickness is equivalent in the Nile delta to 3,600-4,200 ft. of rock 

 above sea-level; in the Niger delta to 5,000-5,500 ft. 



Discussion of results. — -The region of the southeastern Mediter- 

 ranean is held by Suess to be geologically of very recent origin, 

 downfaulted from the continent. The delta of the Nile, much 

 smaller than that of the Niger, is therefore to be regarded as young 

 and may be still increasing in volume. 



The great size of the Niger delta suggests, on the other hand, 

 that it may have reached the limit permitted by the strength of 

 the crust. Subsidence may now intermittently keep pace with 

 deposit. If the i ,000-meter contour has been located correctly, as 

 shown in Fig. 3, it suggests that such may be the case, since it is 



