540 



NEW YOKK STATE MUSEUM 



a transported or sedimentary claj wMcli lias been waslied down 

 to its present resting-place. In transit, the particles have been 

 ground apart naturally, either by rubbing against one another or 

 between the white quartz pebbles scattered so, plentifully through 

 the clay. 



These two clays are practically identical in their composition, 

 as can be seen from the two following analyses. 





Edgar, 

 Fla. 



Dillsboro, 



N. C. 



SiOg 



45.39 

 39.13 

 .45 

 .5^ 

 .'29 

 .83 

 14.01 



43.90 



AloO, 



40.66 



Fe^Os 



CaO 



.14 



MffO 



tr 



■^"-^ ^^ 



Alk 



-16 



H,0 



14 84 









100.61 



K'O.OO 



Physically, there is a marked difference, the Edgar clay being 

 decidedly plastic, the DillsborO' clay being very lean. This plate 

 theory would suggest therefore that plasticity was due to capil- 

 larity, the force of surface tension tending to hold the plates 

 together, but not interfering with their gliding motion. 



The one objection tO' explaining plasticity entirely by the fore- 

 going theory rests on the fact that not all minerals occurring in 

 clay are scaly, and that neither scaly kaolinite nor even scaly mica 

 predominates in all clays. 



Clays may be said to have two classes of particles, viz, plastic 

 and nonplastic, the latter being the sandy grains. 



Olchewsky^ was probably the first to suggest that the plasticity 

 and cohesion of a clay are dependent on the interlocking of the clay 

 particles and kaolinite plates, and in this connection used the 

 briquet method of testing the plasticity, or rather obtaining a 



1 Topf. u. Zieg. Zeit. 1882. no. 29. 



