46 W.S. GRESLEY—CLAY-VEINS INTERSECTING COAL MEASURES. 
topography, etcetera; (7) the general direction of surface drainage. and 
(8) the north line. An examination of the data as plotted in figure 16 
will make this clearer to the reader. 
ORIGIN OF THE VrIN Mareriars 
The character and aspect of the finer clayey material, as also of the 
more fragmentary ingredients of clay-veins, are such as indicate that they 
were principally derived from the walls of the veins. If this were so, 
then these great cracks gap- 
ing upward remained open 
long enough to allow weath- 
ering and the gradual dis- 
integration of their ragged — 
: sides, which bit by bit broke 
off and fell into the clefts, 
carrying down with them 
such additional bits of this 
or that stratum as they 
struck and dislodged. Prob- 
ably running water from the 
rains helped in the filling 
‘process at times, and the 
action of frost may have as- 
sisted; also the presumed 
tendency of the walls to 
swell and come together 
would materially help to 
break up the beds and drop 
in the rock fragments. Prob- 
ably wind-blown sand and 
plant remains, as we have 
seen, fell into the openings. 
The derivation of the in- 
Ercune 10 Diagram dlusuannd ali end sly Clay uc sr a yiUsiye yelled Mnelay mente 
their Relation to other physical Features. 
has already been shown, 
namely, that they consist for the most part of underclays which have been 
softened and squeezed up into open cracks in the overlying coal, though 
the origin of the cracks is more obscure. 
North Line. 
Veins 
ack 
R 
i? 
i 
as 
KR 
R 
8 
5 
ial 
> 
i ee 
al direction of the ‘SU 
rection 
General 
Gener 
So far as we know, nothing has yet been found in a clay-vein except, 
perhaps, the little bits of vegetation above referred to, which can be said 
