THE IMAGINARY RANGE AND TROUGH. 03 



northern edge it will be seen that the depressed tract gives a 

 deflection which is only one-tenth of that due to the alluvial trough 

 but the latter drops in value much more rapidly in amount than tht. 

 former, and in the central and southern portions of the trough, 

 where all but a very few of the stations are situated, the effect of 

 the depressed lower surface of the crust ranges from one quarter 

 to one-sixth of the effect of the alluvial trough on the upper surface, 

 if the two are supposed to be of equal dimensions. 



In one respect this is not likely to be the case, for the boundary 

 of the alluvium does not mark the limit of the depression. To the 

 south of the alluvial boundary the general level of the country 

 continues to rise for some distance and, if the origin of the trough 

 is that assumed by the hypothesis just considered, the width of the 

 depressed lower portion of the crust must be taken at 250 to 300 

 miles. In this case the effect near the Centre of the alluvial area 

 would be reduced by from one-quarter to one-third, and the effect 

 at the southern limit of the alluvium increased in about the same 

 proportion ; the effect, therefore, of the depressed lower portion 

 of the crust may be taken as round about one-fifth of the effect 

 of the alluvial trough over the greater part of the plain. Oulv in 

 the northern part of the alluvium, for a distance of at most GO 

 miles from the northern edge, would the ratio of the two separate 

 effects differ materially from this proportion, and it is just in this 

 region that the deflections give the least satisfactory and certain 

 indications of the form of the floor of the trough. 



The effect of the depressed lower portion of the crust on the 

 force of gravity at the surface may be simply and easily expressed, 

 with sufficient accuracy for present purposes. The defect in mass 

 of the depressed lower portion of the crust is 1 of the whole, that of 

 the alluvial trough is -2, the defect in attractive power is therefore 

 one half as great in the one case as in the other ; but besides this 

 we have to take into consideration the effect of the greater distance 

 from the surface in the former ease, which will have the effect of 

 diminishing the apical angle of the cone covering two circles of the 

 same radius. Taking this radius at 100 miles, and it is needless 

 to take a larger radius seeing that the trough is only 200 miles broad, 

 we find that a disc of the lower surface of the crust would produce 

 an effect of '76 of the amount which a disc of the same total mass 

 would produce at the surface ; and as the mass is taken at one- 

 half, the ratio of the effect of the alluvial trough to that of the 



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