METHODS OF ANALYSIS 657 



inferred from consideration of the physical forces and conditions in- 

 volved. The lightness of the air prevents it from moving pebbles more 

 than about one centimeter in diameter, except on occasions so rare that 

 wind deposits of that coarseness are practically non-existent. Likewise, 

 dust grains measuring less than one sixty-fourth millimeter in diameter 

 are so easily held in the atmosphere that they are readily scattered every- 

 where over the earth's surface and are too widely dispersed to accumulate 

 anywhere as the main ingredient in a distinct deposit, unless it be in 

 some permanently quiet part of the sea. 



For the purpose of obtaining similar measurements on the size of the 

 clastic elements present in other sediments, I later made collections of 

 aqueous and glacial deposits and made similar analyses of these. It was 

 my wish to extend these studies to include the widest possible range of 

 sedimentary conditions; but lack of time for several years to pursue the 

 subject and absence of prospects to follow the work any farther have 

 caused me to decide to publish the data now in hand, however incom- 

 plete. I shall make only a brief discussion of their significance. 



Methods of Analysis 



In making the analyses the clastic elements have been separated into 

 groups of different coarseness. These groups I have called grades. Each 

 grade consists of particles ranging in size between limits marked by two 

 contiguous separations. Tliese separations have been made in a uni- 

 formly decreasing series of diametrical dimensions, so as to give the 

 largest bodies in each grade twice the diameter of the largest bodies in 

 the next finer grade, throughout, from the coarsest to the finest. In 

 general, when the finest grades have been found in quantities amounting 

 to less than one-tenth of a per cent of the whole sample, they have been 

 neglected or marked as a trace. For the sake of convenience, the follow- 

 ing designations for different grades will be used in this paper : 



Diameters in millimeters. 



From— To- 

 Large boulders 256 128 



Medium boulders 128 64 



Small boulders 64 32 



Very small boulders 32 16 



Very coarse gravel 16 8 



Coarse gravel 8 4 



Gravel 4 2 



Fine gravel 2 1 



Coarse sand 1 1/2 



Medium sand 1/2 1/4 



