Li 



MARION ' ' EXPEDITIOX TO DAVIS STRAIT AXD BAFFIX BAY 63 



Twenty-seven samples were studied. There were taken along tra- 

 verse lines of the Marion expedition and afford data for one longi- 

 tudinal section, 200 miles in length, in the central part of Davis 

 Strait off Cumljerland Peninsula, five traverse sections on the west 

 side of the strait between oT'' and 64° X., and single localities from 

 the middle of Hudson Strait, off Cape Farewell and east of southern 

 Labrador. The sediments vary in texture, but they are character- 

 ized by a significant content of faceted, subrounded rock fragments, 

 resulting from ice transportation. The sampling instrument has an 

 internal diameter of 4 centimeters and could not procure pebbles 

 larger than that size. Consequently the total quantity of fragments 

 in the deposits was not ascertainal)le, but from the depth of pene- 

 tration and the small proportion of big pebbles collected, it is prob- 

 able that large rock fragments constitute a very minor part of the 

 sediments. 



IXTERPRETATIOX OF MECHANICAL ANALYSES 



Method of analysis. — The results of the mechanical analyses of 

 the sediments are shown in Tables 2, 3, and 5 and on Figures 47, 48, 

 and 49. The method of analysis was a centrifuge procedure which 

 gives the complete size distribution. - 



The customary separation of the samples into gravel, sand, silt, 

 clay, and colloid is shown in Table 3 and the subdivision of the 

 gravel and sand groups in Table 4. Table 2 shows the fundamental 

 constants of the size distribution. It is impracticable to present the 

 mechanical analyses of so manj^ sediments in histograms, but the 

 important characteristics of the size distribution are indicated con- 

 veniently by three constants, the median diameter, the coefficient of 

 sorting, and the coefficient of skewness or its logarithm. These are 

 given in columns 3, 5, 6, and 7 of Table 2 and on Figures 47 and 48. 



Median. — The median diameter indicates the mid-point of the size 

 distribution. One-half the weight of the sediment is composed of 

 particles larger in diameter than the median, and one-half smaller. 

 The median is the most important single constant for describing the 

 character of a sediment, as it gives a mathematical means of measur- 

 ing variations in texture. Thus, medians from 50 to 1,000 microns 

 indicate sands; from 5 to 50 microns, silts: 1 to 5 microns, clays; 

 and less than 1 micron, colloids. 



Coefficient of sorting. — The coefficient of sorting affords a mathe- 

 matical measure of the degree of sorting of a sediment. It is based 

 on the first and third quartiles, which refer respectively to the one- 

 fourth and three-fourths marks in the size distribution. These are 

 given in columns 2 and 4 in Table 2. Twenty-five per cent of the 

 weight of the sample is composed of particles larger than the first 

 quartile and 75 per cent larger than the third quartile. Thus, by 

 means of the quartiles, the size distribution is divided into four equal 

 parts called quartile intervals. The coefficient of sorting, ^o^ is 

 derived from the formula So=-ylQilQ^. where Q^ and Qz are the 

 first and third quartiles, respectively. If So is less than 2.5, the 

 sample is well sorted; if it is greater than 4.5 the sediment is 

 poorly sorted ; and if it is about 3 the deposit is normally sorted. 



2 P. D. Trask. Mechanical Analysis of Sediments by Centrifuge, Econ. Geol., vol. 25, 

 pp. 581-599, 1930. 



