440 C. T. Clough—The Whin Sill of Teesdale. 
such fissures, and why is there no sign of there being, or ever having 
been, any, except where the Whin is? The only fissures that are 
known in the Dale are either small caves in the limestone, or fault 
fissures, and these last are generally approximately vertical and 
never very wide. The faults too, as has been already said, appear 
to be all posterior to the Whin, so that even the few fissures which 
do exist cannot be called in to help us. 
I wish particularly to guard myself against being supposed to 
consider that all the Whin Sill has been made by the melting down 
of sedimentary beds. Some of the instances that I have noticed 
certainly seem to show that, in places, just the same thickness of 
sedimentary matter has been absorbed into the Whin, as the thick- 
ness of the Whin there present, but of course it does not follow that 
this is so generally, and in considering this question I wish to keep 
within the facts as strictly as possible. I do not say that the Whin 
Sill of Teesdale is wholly made up of sedimentary beds; I only say 
that in certain places certain sedimentary beds have gone to help 
to make it; and because this is so in Teesdale it does not necessarily 
follow that it is so in other districts. There might be particular 
reasons in Teesdale, arising either out of the character of the 
sedimentary beds or the heat of the intrusive mass, which caused 
more beds to be there dissolved up than in other places. 
But if, on independent authority, it can be shown that similar beds 
of Whin in other districts have behaved in the way the Whin Sill of 
Teesdale is said have done, such evidence of course strongly 
confirms that collected here. Now of such confirming evidence there 
is no lack. I have already mentioned an instance from Weardale. 
I am informed by my colleague, Mr. J. G. Goodchild, that the 
excellent sections to be seen on the Pennine escarpment show 
clearly the same absence of disturbance by the Whin, and the same 
apparent loss of sedimentary beds where it puts on. 
In reference to the “ Whin Sills” of Scotland, Dr. James Geikie 
writes to me (Nov. 29, 1878):—*“In the Dalmellington coal-field 
there are some very thick sheets of intrusive basalt-rock, which are 
persistent throughout the whole field. Thus in sinking for the 
black band ironstone the miners may pass through one or more 
sheets, or they may not encounter the thinnest squirt . . . . some of 
these were not less than 30 or 40 feet. Now, it was found that the 
occurrence of one or more of these beds in a shaft added nothing to 
the thickness of the strata. That is to say, that the distance between 
two well-known horizons (such as the black band and the coals at 
©) or 60 fathoms above it) was not increased by the addition of the 
Whin floats. The manager of the ironworks and his engineer assured 
us that this held so true that in sinking a shaft they always got the 
lronstone at the estimated depth, no matter whether Whin-floats 
made their appearance or not. .... I have certainly seen cases 
where beds are without doubt entirely substituted by basalt. Thus 
Seams of coal are occasionally quite eaten up and their place taken 
by * white trap,’ and I have seen the same in the case of a lime- 
stone band.” 
