June, 1906. New Forms of Coxcretioxs — Nichols. 51 



Claystones cannot be boiled to hasten saturation as they dis- 

 integrate to a serious extent. For specimens of this character the 

 use of the air pump is of but little value. This very slow permeability 

 of partially saturated claystones is a necessary consequence of the 

 peculiar mesh-like structure already described by Emerson.* The 

 rate of absorption becomes less as the outer parts become saturated 

 until it is so small that increase in weight of the specimen 

 under treatment is masked or imperceptible for periods as great 

 as 24 hours. The last air of the interior is trapped and can be 

 removed only by solution in the water. This solution is greatly 

 impeded by the slight mobijity of water confined in the capillary 

 spaces so that the dissolved air can be removed by only slow diffusion 

 unaided by convection currents in the water. The density obtained 

 for claystones is therefore less than the true density by a quantity 

 which is greater the thicker the specimen. It is undesirable, however, 

 in order to avoid this presumably small and regular error, to introduce 

 the error due to solution of cement and consequent disintegration of 

 the surface which would arise from too prolonged immersion of the 

 specimen. This latter error which is found to be very large and also 

 very irregular has to be guarded against most carefully. This disinte- 

 gration from the surface of clay stones in water is so great with 

 specimens from some regions that all attempts to ascertain their 

 density proved futile. Where an abundance of material may be 

 sacrificed in the work, pycnometer methods may possibly yield 

 results free from these errors but the experience of the author has been 

 that little dependence can be placed upon pycnometer determinations 

 made upon such small quantities of material as could be sacrificed 

 for this purpose. Hence no such determinations were made. The 

 specific gravities of the claystones examined are tabulated on 

 page 



When the forms of the specimens were compared with their 

 densities an apparent relationship between the density and relative 

 thickness appeared. To properly compare these features a numerical 

 value for the rotundity or flatness of the 'specimen is absolutely 

 necessary. As a suitable expression for the variation of form in this 

 respect the term modulus of rotundity is proposed. The diameter of 

 that circle which has an area equal to the horizontal projection of 

 the concretion is calculated or measured. This divided by the 

 extreme thickness gives the modulus of rotundity, a number which 

 is greater for the thinner forms and which becomes unity for the 



*U. S.Geol. Survey, Monograph XXIX, p. 717. 



