METHODS OF ANALYSIS 



659 



In practice it is necessary to first obtain a series of figures which give 

 merely the relative magnitudes of the percentages sought, and from these 

 the percentages are readily calculated. It has also been found that the 

 best results are obtained not by counting the large number of boulders 

 of the smaller sizes, as shown in the case illustrated above, but to find a 

 chain of ratios between the whole mass of any number of boulders found 

 in each two or three successive grades, and from these to calculate the 

 relative magnitudes of the grades, thus : 



Sizes of boulders in millimeters. 



Ratios found by- 

 counting. 



Keiative 

 agnitude of 

 grades. 



Percentages of 



grades 



to whole sample. 



512-256 .... 



25&-;l28 .... 



128- 64 . . . . 



64-32 . . . , 



Total. 



142 



5 







175 



12 



7 



3.0 



17.8 



71.9 



5.4 



3.0 

 18.6 

 73.3 



5.5 



98.1 



100.4 



The grades having diameters from 16 to one-eighth millimeters have 

 been separated by the use of sieves. The grades below this, consisting 

 of very fine sand and all silt, dust, and clay, have been determined by 

 microscopic measurements, by counts of the particles, and by deriving 

 from these counts a chain of ratios in the same manner as in the case of 

 the boulders and very coarse gravel. When more than one method has 

 been used in the same analysis, the materials treated by different methods 

 have been each separately weighed to determine the correct ratios of the 

 sums of the grades belonging to each kind of material, differently treated. 



In the microscopic work a one and one-half inch eye-piece and a one- 

 sixth inch objective have been used throughout; likewise an ocular mi- 

 crometer ruled in decimal squares. 



Materials Examined 



the three classes of sediments 



The three principal kinds of mechanical sediments examined in these 

 analyses are glacial till, aqueous deposits, and wind deposits. 



GLACIAL TILL 



Strictly speaking, glacial deposits are deposits left from melted ice 

 without having been to the least extent sorted by the water resulting 

 from the melting of the ice. This is the material known as till. De- 

 posits formed by glacial streams or in ice-dammed lakes are really aque- 



