AGE AND ORIGIN OF SLACK-VEINS. : 51 
signs of a rush of water into them are known to the writer. ‘The theory 
that clay-veins occupy deep and narrow channels cut out by running 
water has been advanced, but the appearances do not furnish any proof 
of this. Neither do clay-veins occupy previously existing faults, for if 
they did surely we should find traces of them. Again, is it likely that 
the fissures were formed when the great ice-cap retired northward, and 
are due to elevation of the atfected area on the removal of the weight of 
theice? Had their origin any connection with the formation of the series 
of basins or great rolls running parallel with the Allegheny uplift, the 
clay-veins would surely have a general trend in some particular direc- 
tion, probably roughly parallel with the axis of these basins; but such 
is not the case. 
It would seem, then, that until more facts are acquired the clay-vein 
fissures may at any rate be placed in the category of earthquake phe- 
nomena. 
AGE AND ORIGIN OF THE SLACK-VEINS 
Study of the phenomena has suggested the following theory as to the 
age and origin of the slack-veins: When the filling of the clay-vein fis- 
sures was completed the area embracing them was in the form of a flat 
and warped dome—that is, the surface was more convex than concave, 
and when a subsidence of this clay-veined territory took place, as the 
overlap faults and the slack veins seem to testify, enormous lateral press- 
ure was put upon the clay-veins and produced the effects of compression 
in and in contact with them, as we have seen. When the clay-veins re- 
fused to suffer further squeeze, and the pressure not being exhausted, 
some other part of the strata had to give away; either folding would 
begin or fracture and displacement ensue, or both. What actually hap- 
pened appears to be this: The harder strata broke obliquely across in 
the weakest places, and the beds sliding one on another, with the accom- 
panying folding of the softer clays and other material, produced in course 
of time the veins of soot or slack as we have them today. Probably the 
amount of horizontal displacement in a slack-vein equals approximately 
the sum of the widths of the clay-veins, including spars, comprised in 
the area between the slack-vein—that is, supposing 200 inches equals 
the displacement at a slack-vein, the aggregate thickness of the several 
clay-veins between it and the next slack-vein will be about 200 inches. 
The author regards clay-veins as not only the origin of slack-veins, but 
as having been largely instrumental in regulating the size, courses, and 
number of them in a given area, go that it would appear that if the slack- 
VIlI—Boutn. Grou. Soc. Am., Vou. 9, 1897 
