A STUDY OF COBBLE ABRASION 



511 



Figure 2 shows the plotted average of 85 measurements of rate 

 with cobbles ranging from less than 4 grams to 185 grams. The 

 graph shows very plainly how much more severe the wear becomes 

 as any given piece becomes smaller. This is probably in part due 

 to the increasing jatio of surface to weight. 



Figure 3 shows the size history of a cobble started at 180 

 grams. The rock used in this and all other determinations, unless 



ZOO g/vs 4.22 c/r?. 100 gms 3.35cm. SO gms 2.66cm. 



O o 



ZSgms.Z.lJcm. 10gms.l.S6cm. 3^ms. J.O^cn?. J g/n. .72 cm. 



Fig. 4. — Showing sizes of spheres of limestone (density 2 .65) of different weights. 

 Weights in grams and diameters in centimeters. 



otherwise noted, was Niagara limestone from the outwash gravel 

 at Gary, Illinois. It will be noted that 700 miles of travel reduced 

 the cobbles from 180 to about 10 grams. To aid in visualizing 

 these sizes. Figure 4 may be consulted. 



No extensive comparisons have yet been made between different 

 kinds of rock, but measurements were made on one granite cobble 

 which was more than ten times as resistant as the Niagara limestone, 

 and some limestone has been used which was less than one-tenth as 

 durable, so that there is known to be more than one hundred fold 



