Eolian Action in Southern New England. 69 



Rate and Angle of Wear on Pebbles. — The observations 

 made by W. P. Blake early led to the use of the artificial 

 sand-blast in the arts. The conditions under which erosion 

 takes place in the arts have been studied experimentally 

 by Thoulet and are set forth in his paper, " Experiences 

 synthetiques sue Pabrasion des roches par le sable."* The 

 results of these experiments include observations relating 

 to the rate of wear at certain angles of incidence which appear 

 to me to be significant when compared with some of the peb- 

 bles which I am about to describe. "■ The abrasion," states 

 Thoulet, " is so much the more energetic when the rock upon 

 which it acts is more nearly vertical with reference to the direc- 

 tion of the sand which strikes (horizontally), and it diminishes 

 very rapidly in intensity directly as the inclination becomes 

 lower than 60 degrees." " Oblique incidence too (from 30° 

 to 45°)f increases the rapidity of the cutting effect, probably 

 because the issuing particles of sand are not ' met and their 

 force deadened by the rebounding ones." From these experi- 

 ments we should be led to expect the wear on a nearly vertical 

 surface of a pebble to proceed slowly in nature, to go forward 

 more rapidly after a time when a surface had been worn down 

 to a plane inclined about 30° to the vertical, and again to wear 

 very slowly when the plane had been worn down to an angle 

 of 60 degrees with the vertical. * The result of this varying 

 rate of wear would be that on a pebble plain exposed to sand- 

 blasting, the carved pebbles would soon pass through the 

 maximum rate of cutting and would remain long near the 

 angle of 30° of inclination to the horizon. An examination 

 of several well-facetted pebbles from Martha's Vineyard shows 

 this feature in a marked way. It is necessary, however, in 

 order to ascertain this mature state of erosion, to establish the 

 soil-line or horizontal plane in which the pebble lay at the 

 time of its carving. This can frequently be done with peb- 

 bles found in situ. The smaller pebbles by the erosion of the 

 sand and gravel about their bases have frequently been over- 

 turned, and eroded on all sides. It is evident that where 

 inversion takes place regularly and frequently that facets 

 may fail to develop and that the pebble may slowly waste 

 away in the manner of those described by Gilbert on the 

 plains of Colorado. 



Lithological characters of the pebbles. — There is a certain 

 relation between the sand blown upon the rocks and their 



*Compt. Rend. Acad. Sci., t. civ, p. 381 ; also Annales des mines, Mars-Avril, 

 1887, pp. 199-224. 



f The experiments conducted by Gen. Tilighman in connection with his patent 

 are described by G-. F. Barker in Johnson's New Universal Cyclopedia, vol. iv, 

 pp. 62-65. 



