254 The American Geologist. April, 1895 
in the case of mud cracks, the particles would tend to collect 
about certain centers in the beds, and these centers of adhe- 
sion would increase laterally b}' the attnictionof adjacent par- 
ticles until cracks of var^nng thickness would form between 
the peripheries of adjacent centers of adhesion. The pressure 
of the overlying beds would determine the vertical direction 
of these cracks, and would afford the means for the second 
way, b)'^ which the decrease in the volume of the clays would 
be taken up, viz., by a decrease in the vertical thickness of 
tlie beds. These cracks thus formed far beneath the surface 
were afterwards filled by chalcedony dissolved out of the over- 
h'ing beds by heated waters percolating througli them." This 
explanation would account most perfectl}'^ for the veins which 
cut the strata of the clays in a line oblique to their planes 
of deposition and also the gathering of the veins together in 
common centers, causing the regions to resemble, as mentioned 
by Hatcher, huge septaria. 
Conclnsions: The dikes of mud and sand occupy pre-exist- 
ent cracks which were filled by intrusions from below of wa- 
ter and suspended material. The water was forced into the 
cracks from porous layers either by hydrostatic pressure or 
that of the superincumbent strata, ])robably both combined. 
The cracks are not the results of earth movements. They 
are in all probability both mud cracks and cracks formed by 
segregation of the clays around local centers. 
The veins of chalcedony were formed by the entrance into 
cracks of similar origin as those containing the dikes, of sili- 
cated waters. The cracks were already filled more or less 
completely with water and sand. The thinning out of the 
seams from above downward indicates that the silicated wa- 
ters filtered in from above. 
EDITORIAL COMMENT. 
Secular Changes of Akctic (Jlimate. 
The astronomic theory of the causes of the Ice age, as ad- 
vocated by ('roll, Geikie, and Ball, is examined by Mr. E. P. 
Culverwell in the Geoloqical Jfagazine for Januar}" and Feb- 
