JUNE, 1906. New Forms or ConcretTions—NICHOLS. a 
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 mobility 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. 
