1893.] The Titanotherium Beds. 209 
with the same materials as the overlying beds; for when the 
waters again covered this region, the mud cracks would be 
immediately filled by sedimentation and with the same mate- 
rials that now compose the overlying beds. It has sinee 
occurred to the writer, that these cracks were not made while 
the particular strata in which they now appear occupied the 
immediate bottom of the lake, but after the overlying beds 
were deposited. The extreme fineness of the particles form- 
ing the clays of the Titanotherium beds in those places where 
these veins occur is evidence that the clays were deposited by 
a slow process of sedimentation in still waters. The bottom 
of a lake where such materials were being laid down would 
consist for several feet of a very thin mud or ooze. This would 
gradually become firmer toward the bottom as deposition con- 
tinued, but would still mechanically retain a considerable per 
cent. of water. Later, when the entire overlying series of 
strata were deposited and the region brought permanently 
above the water level this imprisoned water would gradually 
disappear, by filtration or otherwise, aided perhaps by the 
pressure of the superincumbent beds. This loss of moisture 
in the clays would diminish their volume and bring about a 
readjustment of the particles composing them. The decrease 
in volume would be taken up in two ways: First, as in the 
case of mud cracks, the particles would tend to collect about 
certain centers in the beds, and these centers of adhesion 
would increase laterally by the attraction of adjacent parti- 
eles until cracks of varying thickness would form between 
the peripherys of adjacent centers of adhesion. The pressure 
of the overlying beds would determine the vertical direc- 
tion of these cracks, and would also afford the means for 
the second way, by which the decrease in the volume of Ne 
clays would be takem up, viZ., by a decrease in the verti 
thickness of the beds. These cracks, thus formed far beneath 
the surface, were afterward filled by chaleedony — 
out of the overlying beds by heated waters perco EA 
through them. Occasionally other minerals, as ordinary cal- 
cite and its less common, variety known as Iceland spal ars 
found in small cavities in these vains.. Localities containing 
15 
