﻿Geological Society of London. 37 



A review of the literature on the subject was given by the 

 authors, showing that the opinions of geologists are very much 

 divided as to the nature of the Whin Sill. But amongst the prac- 

 tical miners of the north of England there are very few who will 

 admit any doubt that the Whin lies evenly, and at one constant 

 horizon, amongst the strata. Clear cases to the contrary are looked 

 upon as merely local variations, possibly due to successive eruptions 

 of subm-arine lava. The Whin Sill serves them as a definite line, 

 and the limestone next above it is always called the " Tyne Bottom 

 Limestone." The question is thus of considerable economic im- 

 portance. It is also of interest in reference to the volcanic history 

 of Britain and to classification. 



Prof. Phillips took the Whin Sill as the base of the Yoredale 

 Series ; the Great Limestone he regarded as its top. But the work 

 of the Geological Survey has shown that the Whin Sill lies at dif- 

 ferent horizons 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 which forms the top of that series. 



With the disappearance of the supposed base-line of the 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 character. 



The authors traced the Whin 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 fi-e- 

 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. 



