348 C. Davison — Least Age of the Stratified Bocks. 



I am far, however, from wishing to lay too much stress on this 

 oase of Lake Bonneville. Its outline is intricate and extensive, and 

 it may be that a more exhaustive survey would modify the results 

 arrived at by Mr. Gilbert. It is quite possible also, as he himself 

 remarks, that the deformation of the old shores may be due to sub- 

 terranean movements proceeding from an entirely different cause. 

 But, so far as the Survey has gone, it certainly fails to show that 

 there was any notable gravitation of the lake-waters towards the 

 liigh mountains on the East, such as might have been expected upon 

 the hypothesis that a strong attractive force was exerted by these 

 mountain-masses. 



Assuming, for the present, that the bottom of the lake has actually 

 risen 168 feet in consequence of being relieved from the pressure of 

 1000 feet deep of water, we may use this result to ascertain what 

 might be the effect joroduced by the disappearance of a like depth of 

 ice. Taking the specific gravity of glacier-ice at 875 as compared 

 with water at 1000, the result would be a rise of 147 feet — or say 

 in round numbers 150 feet for a thousand feet of ice. This would 

 agree tolerably well with some of the changes of level that have 

 been observed in regions formerly occupied by large glaciers. 



IV. — On a Method of Determining a Lower Limit to the Age 

 OE THE Stratified Eocks. 



By Charles Davison, M.A., 

 Mathematical Master at King Edward's High School, Birmingham. 



1THE following is an attempt to determine a lower limit to the 

 I time required for the accumulation of the stratified rocks. It 

 is not supposed to be an accurate estimate, for the data on which it 

 is based are confessedly imperfect ; but it is thought that the method 

 employed may be of some little use when more exact determinations 

 can be made of the physical constants involved. 



The first part of the method is founded upon the observed fact 

 that most, if not all, of the stratified rocks, throughout the period of 

 their formation were deposited in shallow water ; the latter part on 

 the law of secular cooling deduced by Sir W. Thomson for the case 

 of a solid earth, though not on the value of the initial temperature 

 used by him. As a consequence of secular cooling the outer parts 

 of the earth's crust are folded by lateral pressure, the rocks above 

 the troughs gradually subsiding as the troughs of the folds deepen, 

 by reason of the weight of the rocks, and of the sediment, if any, 

 deposited above them. 



2. The fact that stratified rocks are chieflj' shallow-water deposits 

 indicates that the rate of sedimentation is generally, and on the 

 whole has been, greater than the rate of subsidence.^ Now, the 

 sediment brought down by a river is used : first, for keeping the 

 level of the delta-surface close to the level of the sea ; secondly, for 

 extending the delta laterally by means of the surplus sediment. 



^ The rate of subsidence at any time is here measured with respect to the level of 

 the sea at that time. 



