were run through the carbon analyzer. The difference between the total 

 carbon content and the carbon remaining after treatment with weak acid 

 was the carbonate content of the sediment. 



b. Statistical Analysis . The following four statistical para- 

 meters were calculated for each sample: Mean grain size, standard 

 deviation, skewness, and kurtosis. The formulas used were from Folk 

 and Ward (1957). 



(1) Mean Grain Size . The mean grain size is the mean 

 calculated by the formula (<j>16 + 4>50 + 4>84)/3. The three phi (4>) units 

 are percentiles taken from a plotted cumulative curve (Tyle standard 

 screen scale). Corresponding size limits in phi units and millimeters 

 in relation to particle-size classification are shown in Table 2. 



(2) Standard Deviation . The standard deviation is a measure 

 of the average spread of the curve about its central tendency and 

 indicates the degree of sorting of particles in the sample. The 

 formula expressing this value is: 



4>84 - $16 . (|)95 - (j>5 



4 6.6 



This formula includes 90 percent of the distribution and, therefore, 

 provides a wide-ranging measure of sorting. Folk (1964) identified 

 various standard deviations in relation to the degree of sorting 

 (Table 3). 



(3) Skewness . Skewness indicates the displacement of the 

 median from the mean, and is independent of sorting. For example, a 

 symmetrical curve would have a skewness of zero; with an excess of 

 fine material, the sample would be skewed right or positive; a sample 

 with an excess of coarse sediments would be skewed left or negative 

 (Table 4). The formula is: 



4>16 + 4>84 - 2<|)50 + 4>5 + £95 + 2<t>50 

 2(4)84 - 4)16) 2(4)95 - <f>5) 



(4) Kurtosis . Kurtosis is a measure of the ratio of the 

 sorting in the extremes of the distribution compared with sorting in 

 the central part (Folk and Ward, 1957). If the data can be plotted 

 as a straight line on probability paper, a normal distribution with a 

 kurtosis value of 1 is indicated. A departure from the straight line 

 indicates a departure from normality and changes the kurtosis. If 

 the central part is better sorted than the tails, the curve is 

 excessively peaked or leptokurtic; if the tails are better sorted than 

 the central part, the curve is flattened or platykurtic. The formula 



26 



