No. 14.] MECHANICAL COMPOSITION OF SAMPLES. 25 



ing Va and 2 mm., Vi and 4 mm., etc., will be as nearly as possible equal. 

 A single operating force — in this case the velocity with which the water 

 moves at a particular place — will thus always make itself apparent in a 

 perfectly regular, symmetrical curve — the so-called curve of probability — , 

 which will be more arched or flatter according to whether the force in question 

 has operated more or less regularly and exclusively. If we see that the curves 

 take another form, we may conclude that some factor or other has asserted 

 itself, which must be calculated separately in each case. If, for instance, the 

 right side of the curve is greater than the left, coarser particles must have 

 become mingled in the sample by some special natural agency; accidental 

 circumstances in the principal depositing force cannot by themselves have pro- 

 duced such irregularity. 



As regards the bottom-samples, the curves that demonstrate their mecha- 

 nical nature, will have many varied and peculiar forms. This shows directly 

 that a number of different natural forces have asserted themselves at the time 

 of deposition. The bottom-samples on the whole, belong to the less regular 

 deposits; the numerous alternations in the sea, in wind, temperature, and 

 currents, the many different sources whence the constituents originate, 

 and further the previously mentioned, peculiar condition that clay presents in 

 salt water, all cooperate in making the deposits irregular. The curve for each 

 separate sample shows directly how great this irregularity is, and thus sup- 

 plies a means of finding out to some extent, to what degree the various forces 

 have asserted themselves in the deposition of the sample. 



The curves were constructed in the following manner. First, on the axis 

 of the ordinate, figures are placed indicating the percentage of grains of each 

 separate size. The highest value that it has ever been necessary to employ 

 with these samples is somewhere about 30. In very homogenous rocks, it 

 will rise somewhat higher, though very seldom above 70 or 80. In the next 

 place, the values of the sizes of grains are placed on the axis of the abscissa, 



1 mm. being taken as the starting-point, and the quotient 2 as the unit. The 

 area of the curve situated over each single unit indicates the quantity of the 

 corresponding size of grains. The scale is thus unlimited on both sides. 

 In these samples it has not been necessary to go to greater values than 



2 mm., and the lowest value given is -rhj == ^TT- Among the values 

 employed in washing, - 05 and 001 mm. do not fit directly into this 



