MICROSCOPICAL CHARACTERS OF THE WHIN SILL. 641 
Topiry and Lesour. On the intrusive Character of the Whin Sill of Northum- 
berland. Q.J.G.S. vol. xxxiii. p. 406. 1877. 
Lespour and Fryar. Onthe Harkess Rocks os Bamburgh. Trans. N. of 
England Inst. of Engineering, vol. xxvi. 1877. 
Lesour. Outlines of the Geology of Nor Wakil anal Newcastle, 1878. 
Geological Map of the County of Northumberland. 
Borney, T. G. New Theory of the Formation of Basalt. Proc. Geol. Assoc. 
vol. vii. p. 104. 
TE term Whin Sill is applied in the north of England to the 
intrusive sheet of basic igneous rock which forms such a marked 
feature in certain portions of the Lower Carboniferous district of 
Durham, Cumberland, and Northumberland. It is unnecessary, on 
the present occasion, to describe the stratigraphical relations of the 
rock, because full details are to be found in the papers cited above, 
and especially in those by Prof. Sedgwick and Messrs. Topley and 
Lebour. For some time there was a discussion as to whether the 
rock was intrusive or interbedded, but this discussion may be 
regarded as having been closed by the publication of the able paper 
by Messrs. Topley and Lebour. It is now admitted on all hands 
to be intrusive. To give some idea of its extent, I may mention 
that it is exposed as an inlier in Teesdale for a distance of many 
miles. It reappears in the Cross-Fell escarpment in Cumberland, 
and may be traced thence, with slight interruptions, across the 
county of Northumberland, to the sea-coast at Dunstanburgh, 
following in a general way the strike of the beds with which it is 
associated. As the strike bends round to the N.W. in the northern 
portion of Northumberland, the Whin Sill reappears on the coast at 
Bamburgh, and may be traced from this point to Kyloe, where it is 
last seen. 
The distance from the point in the Cross-Fell escarpment, where 
it first appears, to Dunstanburgh is about 60 or 70 miles. Speaking 
of its development in Northumberland, Prof. Lebour says * :—“ Its 
thickness varies very greatly, being scarcely 20 feet in places, and 
150 feet in others. On an average it is from 80 to 100 feet thick.” 
Inasmuch as the outcrop follows on the whole the strike of the beds, 
we may reasonably infer that the horizontal extension of the sheet 
is very great, amounting, in all probability, to hundreds of square 
miles, and that it underlies a large portion of the counties of 
Durham and Northumberland. It is an unfortunate circumstance 
that we are unable to speak definitely as to the period of its erup- 
tion. It is later than the Lower Carboniferous, and earlier than 
most of the faults of the district +; but whether it is pre-Permian 
or post-Permian must remain, for the present, an open question. 
It would be a very curious circumstance if the Whin Sill should 
prove to be contemporaneous with the diabase of the Eastern States 
in these analyses and was therefore weighed in part with the SiO,, and in part 
with the Al,O, and Fe,O,. The excess of potash over soda is a remarkable 
feature in these analyses. 
* Outlines of the Geology of Northumberland, p. 55. 
+ ‘“ Intrusive Character of the Whin Sill,” Topley and Lebour, Quart. Journ. 
Geol. Soe. vol. xxxiii. p. 418, 
