﻿Yol. 64.] QTJANTITATITE METHODS TO THE STUDY OF ROCKS. 177 



Conclusions to be drawn from tlie Change in the Angle 

 of Rest in Various Rocks. 



When material like sand or oolitic grains is drifted along tlie 

 bottom to where the velocity of the current is so reduced by 

 increased depth that the material can be no longer washed along, 

 it falls down on a slope at the angle of rest. By this means a bed 

 may have been formed several hundred yards long, and the angle of 

 rest can easily be determined, allowance of course being made for 

 the true dip of the strata. This sort of bedding should be carefully 

 distinguished from irregular deposition, which may very properly be 

 called false bedding. As it is formed by the drifting along of 

 the material, I have always called it drift-bedding. 



The value of the original angle of rest can usually be estimated 

 from the nature of the deposit ; but, in many cases, it has been 

 subsequently much reduced by chemical or mechanical changes in 

 the rock, and the alteration in the thickness of the bed can be 

 learned by comparing the values of the tangents of the original 

 angle of rest and of the present angle. In the case of irregular or 

 thin drift-beds or drifted ripples, the original angle of rest may have 

 been materially less than normal, because of the current sweeping 

 down the slope, and thus it could not be relied on. There is also 

 doubt regarding the original value when, as in some limestones, 

 the rock has been so changed that the character of the material 

 when deposited is imperfectly known. I much regret that I did 

 not see the importance of these facts in years gone by, and did 

 not measure the angle of rest in many cases suitable for these 

 calculations ; but fortunately I have sufficient data to show the 

 kind of results that could be obtained by a more complete appli- 

 cation of this method, now that all parts of the subject have been 

 more developed. 



Freshwater Limestone^ Binstead (Isle of Wight). 



The angle of good drift-bedding was found to be as a mean about 

 24° ; and, assuming that originally it was about 34°, the reduction 

 in thickness has been from 100 to 66, or a contraction of 34 per cent., 

 which to a large extent may have been due to the iilling-up of 

 original cavities by carbonate of lime derived from aragonite-shells 

 in close proximity. 



Lower Greensand near Folkestone. 



I measured several good cases, and found the mean angle of 

 drift-bedding to be close on 20°, in sand which would originally 

 have had an angle of about 34°. This would indicate a contraction 

 of from 100 to 54, or of 46 per cent., which must to some extent 

 be due to the filling-up of the interspaces, but also to removal of 

 material by solution. 



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