CHARACTER OF THE WHIN SILL OF NORTHUMBERLAND. 419 



if the rocks have heen much disturbed and faulted before the deposi- 

 tion of the Permians. In this case the tendency to keep along the 

 lines of bedding, would doubtless be greatly lessened. 



The question turns upon the amount of negative evidence. If we 

 find intruded sheets of trap in Carboniferous rocks in the neighbour- 

 hood of unconformable Permians, and never find the trap entering 

 the latter, there is presumptive evidence in favour of the view that 

 the trap is pre-Permian. But this is only negative evidence at best, 

 and, however much there may be of it, it can never amount to proof; 

 nor can it be allowed any weight as against a single fact on the 

 other side. 



But in Staffordshire and Ayrshire we are not dependent only on 

 such negative evidence ; there many faults which throw Coal 

 Measures and trap alike, do not throw the Permians. In these cases 

 the evidence is positive, and of great force. It gives the age of the 

 trap with sufficient clearness : the trap is newer than the Coal 

 Measures in which it lies ; it is older than the faults which throw it ; 

 and these faults are older than the Permians of those areas. 



The Whin Sill of the Penine escarpment approaches to within 

 a mile or two of the Permians of the Vale of Eden ; and here an in- 

 teresting question arises. The Carboniferous rocks are well developed 

 on the west of the Penine fault ; but Mr. Grood child informs us that 

 in no case, so far as is yet known, do they contain any beds of trap, 

 although the beds in which Whin Sill might be expected to occur 

 are there seen. Two explanations of this are possible. It may be 

 because the beds to the west of the escarpment were faulted down 

 to the west before the intrusion of the Whin Sill, and the west- 

 erly continuation of the Sill, in beds newer than the Carboniferous, 

 has been removed by denudation : in this case the Whin Sill would 

 be post-Permian. 



Another explanation is this : — The W'hin may have been injected 

 before the faulting of the beds, but, because of its thinning to the 

 west, it did not reach so far as the Carboniferous beds which lie on 

 the west of the Yale of Eden. 



The latter is the more probable explanation ; or at least it is the 

 one which may most safely be suggested. The westerly thinning of 

 the Whin Sill, from Teesdale to the Penine escarpment, is a fact 

 which is well proved. The thinning of the Sill to the west is seen 

 in the small transverse valley at High-Cup Nick. The same occurs 

 with the " Little W T hin Sill," of Weardale. This is 20 feet or more 

 thick on the east, but it wedges out entirely about 3 miles to the 

 west. 



As there are no " necks " on the west of the line of outcrop of 

 the Whin which can have served to give vent to the trap, we must 

 conclude that it came up in some area to the east of its present 

 outcrop. The westerly thinning of the Whin in certain districts also 

 points to the same conclusion. 



We have, then, no good evidence in our district as to the age of 

 the Whin Sill ; but by comparison with other districts it appears 

 safer to regard it for the present as probably of late Carboniferous, 



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