THE IMAGINARY RANGE AND TROUGH. 



59 



need to carry the depth at the northern limit outwards at a lesser, 

 and then upwards at a steeper slope to the southern margin as is 

 shown in fig. 5 (page 58). From this it will be seen that the 

 effect of introducing the concept of compensation would not mate- 

 rially alter the conclusions drawn from the observations if it is 

 not considered ; the exclusion of compensation would merely reduce 

 the depth by about one quarter in the deeper and by nearly one 

 half in the shallower southern part of the trough, and slightly 

 modify the general form of the cross section. 



The gravitational effect of the alluvium is easily dealt with ; 

 the anomaly having been calculated on the assumption that the 

 whole of the alluvium consisted of average rock, there would be, 

 apart from other causes, an apparent defect of gravity, due to the 

 diminished attraction of the alluvium, whose density is only about 

 four-fifths of the rock by which it was assumed to be replaced. As 

 a layer of average rock of indefinite extent exerts an attractive 

 force equivalent to -0033 dyne for each one hundred feet of thick- 

 ness, it follows that the gravitation effect of a depth of 15,000 ft. of 

 alluvium, of sufficient extent, would exert an effect of — 100 dyne. 

 If the boundary of this trough were vertical the effect at the boundary 

 would be exactly half this value, and at intermediate distances 

 the effect would be as shown in table No. 18. 



Table 18. — Gravitational effect of the defect in density of a vertical- 

 sided trovgh of alluvium, 15,000 ft. in depth, and density S of 

 average rock. All values negative and expressed in dynes. 



Station. 



— 5 



— 4 



— 3 



— 2 



— 1 



— -5 







+ 

 + 

 + 

 + 



+ 



♦6 

 1 

 2 

 3 

 4 

 5 



Defect of gravity. 



•000 

 •001 

 •002 

 •003 

 •005 

 •Oil 

 •050 

 •089 

 •095 

 •097 

 •099 

 •100 

 •100 



An examination of this table shows that the effect of the limi- 

 tation of the trough is barely noticeable at distances of over ten 



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