616 MR. F. A. STEART ON OVERTHRUSIS AND OTHER [ Nov. 1902, 
exactly the same circumstances were met with throughout the entire 
area: that is, the same irregular and thin coal and the same intervals 
of dead ground vertically below the corresponding accidents in the 
seam above. If these abnormal areas were deposited as we find 
them, it is surely remarkable for these irregular deposits to occur in 
almost the same corresponding place in different seams of coal. 
The flat roofs of the coal-seams within the areas of dead ground 
are invariably striated, the strie as a rule running north and south. 
The termination of coal against dead ground is always smooth 
and polished, and often rounded. The same fact is noticed in the 
case of the termination of thick coal. 
On the Slyving Vein, which has about 1 inch of inferior cannel 
on the top of the seam, it has often been observed, where thick coal 
has occurred, that the cannel is found on the top of each layer of 
coal. If the thick coal had been deposited as such, it is curious 
that a layer of cannel should occur on the top of each piece of 
coal. 
Dead ground is very rarely met with in the seams of the 
Farrington Series. 
My view is that all these effects have been caused by the gradually 
increasing amount of movement of the strata from the top seam 
(or Great Vein) to the bottom seam (or Bull Vein), after all had 
been deposited. Evidence has already been given of the increasing 
amount of overthrust of each seam of the series in the overthrust- 
faults which have been noticed. JI consider that the effect in 
the strata in descending order was to cause friction» between the 
beds, which must have moved differentially. ‘This friction must 
have become more effective along the lines of the coal-seams and 
underlying soft shales, these being weak points; and here the effect 
has been to scrape the coal away from the under side of the seam as a 
rule, causing areas of ‘ thin coal,’ or sometimes entirely removing 
the coal, leaving only the soft shales, when it is known as ‘ dead 
ground.’ In other localities the same action has packed one and 
the same coal in several layers one under the other, and has caused 
the polished and rounded ends of the same as shown. This differ- 
ential action would account for similar circumstances vertically under 
each seam, and would account for the striz on the roofs of the 
seams which are always so abundant in these areas. It would 
account for the coal-seams nearly always thinning-out from the 
under side upward, for although the roof and floor have both moved 
in the same direction, still the floor has moved farther or at a greater 
rate than the roof, and has therefore rubbed against the under side 
of the coal, which (as a result) has thinned out, or been entirely 
removed. 
Study of the overthrust-faults may also explain the reason 
why the coal-seams of the Farrington Series remain so much more 
uniform in thickness than those of the Radstock Series, and their 
comparative freedom from dead ground. For we have noticed 
