368 A. C. LANE WEIGHT OF SEDIMENTARY ROCKS 



balanced at the surface, or, as it is said, that the isostatic compensation 

 is at depth. 



A second solution supposes that 60 kilometers below sealevel is a level 

 such that everything beneath can be treated as concentrated at the earth's 

 center, and that above every column, no matter how long or short, whether 

 under plain or mountain or under the sea, has the same pressure ; so that 

 a longer column must differ from the standard column, — which for ma- 

 terial above sealevel is supposed to have a volume weight of 2. 6 '7, — by a 

 uniform amount. This would be the effect if all was floating on a fluid 

 crust of this thickness. 



The third solution supposes that this crust is 113 kilometers, 70 miles 

 thick. 



The fourth solution, Bouguer's, supposes that the earth is rigid; that 

 the crust, we may say, is of infinite thickness, and that local masses above 

 sealevel weigh 2.67 tons per cubic meter and attract accordingly. 



They give a series of maps showing how the observed attractions com- 

 pare with those which should be expected on the four various suppositions. 

 They give the corrections also for other depths (42.6, 56.9, 85.3, 113.7, 

 127.9, 156.25, 184.6 kilometers depths). The solution for a crust 60 

 kilometers thick makes the differences between the real and the calculated 

 gravity least, especially in flat regions and in the valleys of mountainous 

 regions. 



So far as pressure downward is concerned, however, it makes no differ- 

 ence with these columns whether they have a uniform weight or one 

 unequally distributed, so long as the total weight is kept the same. If the 

 top is extra light with sediment or water, it is quite possible that it is 

 balanced by extra weight at the bottom ; but a column of uniform weight, 

 and one of light top and heavy bottom, that weighs the same below will 

 not have the same attraction on the top so far as their effect on gravity 

 at the station of observation is concerned. 



Bowie has given on page 73 a table from which may be computed with- 

 out much difficulty the kind of effect produced. The observed gravity 

 tends to be less on the Cenozoic strata, where the stations are very likely 

 underlaid with strata which begin with a weight from two tons per cubic 

 meter down, and a couple of thousand feet of such light strata would 

 account for the effect produced. 



But we have also seen that strata with gas in them are 10 to 20 per 

 cent lighter ; and this may help to cause a set of differences characteristic 

 of gas. In that case observations of isostatic anomalies might have value. 



Bowie's suggestion that the cause of certain positive anomalies — that 



