362 
GEOLOGY OF THE FOURTH DISTRICT- 
same blue color as those beneath, and this change takes place after sufficient exposure. This 
may be made apparent by the following illustration : 
172. 
Bank of blue and yellow clay , Dunkirk, Chautauque county. 
The lower deposit is a blue clay, with small pebbles or gravel sparingly intermixed ; the 
upper part is a yellowish brown clay, with veins of the same penetrating the blue clay in the 
manner represented in the woodcut. The upper part is distinctly mingled with gravel, as 
well as the veins, while the blue appears a more pure clay; but this appearance is fallacious, 
for on examining the blue portion, it is found to resemble the upper precisely, except in color. 
Farther examination also proves that these veins are merely apparent, being caused by the 
discoloration of the blue clay from the percolation of water. The water sometimes penetrates 
laterally, and produces beautiful ramifications of the yellowish veins. In this example the 
process is perceptible, and the passage of water along these lines can be witnessed at any 
time. In many other cases the cause is not so apparent, though in a large number of instances 
I have been able to detect the same agency, attended with similar results. 
This is a subject well worthy of attention, not only among superficial deposits, but among 
the older rocks; for these have often been near the surface or above water before their in¬ 
duration. How far it may explain the change or alteration of color in successive strata, or 
the apparent veins of segregation in some of our rocks, I am not now prepared to decide ; but 
I have seen numerous examples apparently due to changes of this kind, which have operated 
before the strata became fully consolidated. 
In the clays, it of course happens that the yellowish deposits are subsequent to the blue, 
and they are often more pervious to water from an intermixture of sand; but the limit of the 
change in color will be found at the depth to which surface water penetrates. The lower 
clays are the more purely argillaceous, and therefore less pervious to water; consequently 
this portion of the deposit retains its color, while the more sandy parts are rapidly changed. 
