37 
Geological Society of London. 
A review of the literature on the subject was given by tbe 
authors, showing tbat tbe opinions of geologists are very much 
divided as to tbe nature of tbe Wbin Sill. But amongst tbe prac- 
tical miners of the north of England there are very few who will 
admit any doubt that the Wbin lies evenly, and at one constant 
borizon, amongst tbe strata. Clear cases to tbe contrary are looked 
upon as merely local variations, possibly due to successive eruptions 
of submarine lava. Tbe Whin Sill serves tbem as a definite line, 
and the limestone next above it is always called tbe “ Tyne Bottom 
Limestone.” Tbe question is tbus of considerable economic irn- 
portance. It is also of interest in reference to tbe volcanic history 
of Britain and to Classification. 
Prof. Phillips took the Wbin Sill as tbe base of tbe Yoredale 
Series ; the Great Limestone be regarded as its top. But tbe work 
of the Geological Survey bas sbown tbat the Wbin Sill lies at dif- 
ferent borizons in different places ; sometimes it even lies above the 
Great Limestone itself. In other words, the Whin Sill, which is 
supposed to marJc the base of the Yoredale Series, sometimes lies above 
the limestone ivhich forms the top of that series. 
With tbe disappearance of the supposed base-line of tbe Yoredales 
goes also any good reason for drawing a line here at all. The so- 
called “Tyne Bottom Limestone” cannot be traced definitely through 
Northumberland, and the beds above and below this horizon have 
the same general cbaracter. 
The authors traced the Wbin Sill through Northumberland, as far 
north as Dunstanborough Castle, showing the varying positions at 
which it occurs in the Limestone series, and noting points of 
interest in some of the sections. The Whin shifts its position 
amongst the strata to the extent of 1000 feet or more. It fre- 
quently comes up in bosses through the bedded rocks, and bakes the 
beds above it quite as much as those below, especially when those 
beds consist of shale. 
As to the age of the Whin Sill, nothing definite can be said. It 
is frequently thrown by faults and lodes. There is no certain case 
of its being unaffected by faults which throw the neighbouring 
rocks, although there are a few doubtful cases which seem to point 
in this direction. As the Whin Sill does not approach the Permian 
area of Durham, the fact that some of the faults there are believed 
to be pre-Permian cannot be applied as a test of age in this case. 
In other districts in Britain in which intrusive basaltic sheets 
occur amongst the Carboniferous rocks, there is good reason to 
believe that in most cases they are pre-Permian, or at least pre- 
Triassic. Whether or not this be the case with the Whin Sill 
cannot be determined. No light is thrown on this question by the 
composition of the rock. Mr. Allport has shown that it resembles, 
in all essential characters, the basalts of other Carboniferous districts, 
some of which are possibly contemporaneous, some certainly in- 
trusive. 
