THE PHYSICAL PROPERTIES OF CLAY. 83 



believed that the increased plasticity of the sedimentary clay is 

 due to this cause. This conclusion is in accord with facts noted 1 

 by the former State Geologist, Prof. Geo. H. Cook, who found 

 that by rubbing a mass of kaolin in a mortar the bunches of 

 kaolinite plates were easily broken apart and that the mass after- 

 wards showed increased plasticity. From what has been just 

 said it must not be inferred that all residual clays are of low 

 plasticity, for some are just the opposite. 



If an exceedingly plastic clay is examined under the micro- 

 scope, it is found that in addition to the regular particles of 

 either a scaly or irregular shape, there are often a large number 

 of extremely minute, apparently spherical, structureless particles, 

 which may be of colloidal character. 2 It seems to the writer that 

 a mixture of the mineral particles and these spherical particles 

 might develop a high degree of plasticity. 



This same view has been independently expressed by A. B. 

 Cushman. 3 



TENSILE STRENGTH. 



The tensile strength of a clay is the resistance which it offers 

 to rupture or being pulled apart when air dried. It is an import- 

 ant property by virtue of which the unburned clay ware is able 

 to withstand shocks and strains of handling and the shrinkage in 

 drying. Through it, also, the clay is able to 1 carry a large quan- 

 tity of nonplastic material such as flint or feldspar. 



There may be some relation between the plasticity and the 

 tensile strength of a clay, but it is neither a constant nor a simple 

 one. While high tensile strength and high plasticity often go 

 together, there are marked exceptions, A clay low in tensile 

 strength may yet have high plasticity, as is seen in the case of 

 some New Jersey clays, and, on the other hand, however, a clay 

 of only moderate plasticity may have high tensile strength. 



1 Report on Clays, 1878, pp. 281, 287. 



2 Maryland Geol. Surv., Vol. IV, p. 251. 



3 Jour. Amer. Chem. Soc, XXV, No. 5, 1903. 



