104 proceedings of the amherst meeting 



Sectional Meeting of Wednesday Afternoon 



titles and abstracts of papers presented before the afternoon 

 session and discussions thereon 



The sectional meeting for the reading of stratigraphic papers convened 

 in the afternoon, under the chairmanship of Vice-President Keith, while 

 J. J. Galloway served as secretary. The following program was presented : 



VALUE OF THE PHYSICAL CHARACTERS OF SAND GRAINS IN THE 

 INTERPRETATION OF THE ORIGIN OF SANDSTONES 



BY J. J. GAIXOWAY 



(Abstract) 



The study of .sandstones may be approached from six different directions, 

 namely, the physical characters of the grains, mineralogical character of the 

 grains, structural features of the rock, paleontology of the rock, stratigraphic 

 relations, and paleogeographic probabilities. 



The physical characters of the grains include: (1) limits of size and aver- 

 age size; (2) proportion of different sizes (mechanical analysis) ; (3) per- 

 centages of angular, subangular, semiround, subround, and round grains ; (4) 

 proportions of rounding in the different minerals; (5) lower limit of rounding; 

 (6) character of the surface of the grains; (7) coating on the surfaces of the 

 grains. 



Extensive experiments were conducted and observations made to determine 

 the value of the physical characters of sand grains in the interpretation the 

 agent of clastation, agent of transportation, and agent of deposition of sand- 

 stones. Limits of size and mechanical analysis were found to be of little value. 

 When more than 50 per cent of the grains are well rounded, the sand is 

 eolian, unless the sand is calcareous, in which case no agent of abrasion is 

 indicated; if less than 50 per cent of the grains are well rounded, no agent of 

 abrasion is indicated. If feldspars, garnet, zircon, mica, cassiterite, monzonite, 

 or other minerals than quartz are rounded most, water abrasion and deposi- 

 tion is indicated. Grains of all common constituents of sands were rounded 

 experimentally by mutual abrasion in water at velocities of four miles an hour 

 or less to 0.01 millimeter in diameter; the lower effective limit of rounding by 

 wind was found to be 0.03 millimeter, and the lower effective limit of rounding 

 in water was found to be 0.05 millimeter in diameter. The presence, therefore, 

 of a few round grains less than 0.1 millimeter in diameter in a sandstone is 

 without significance. Wind-abraded grains of sand are "frosted" like ground 

 glass; water-abraded grains are polished and glassy. A coating on grains indi- 

 cates atmospheric weathering or abrasion in air at low velocities. 



By a consideration of the mineralogical characters of sandstones, the struc- 

 ture of the grains (clastic or built up), the shape of the grains, and the sur- 

 faces of the grains, the agents involved in the formation and transportation 

 of the materials of friable sandstones or sands can be accurately determined. 



Read from manuscript. 



