420 W. TOPLET AKV G. A. LEBOTJR ON THE INTRUSIVE 



or possibly of early Permian age *. Prof. Sedgwick believed that 

 the "Whin Sill had been injected before the deposition of the Mag- 

 nesian Limestone. Mr. Tate, chiefly from his interpretation of the 

 evidence of faults and lodes, regarded it as " subsequent to the Car- 

 boniferous, and prior to the Triassic era." 



In this view of the case, it is important to consider the amount of 

 consolidation which the beds had undergone, and the thickness of 

 beds deposited previous to the intrusion of the Whin. The Carboni- 

 ferous rocks of other districts seem often to have been generally more 

 altered and disturbed by the intruding trap than do those of our 

 area ; they seem to have been less fully consolidated, and to have 

 presented less resistance to the eruption of the trap. 



So far as we can judge, the rocks of our area had been fairly well 

 consolidated (the clayey sediment had been compressed into shale, 

 and the carbonaceous matter had been fully changed into coal) before 

 the injection of the trap. There must have been a very great thick- 

 ness of rock overlying the Limestone Series at that time ; else Ave 

 should find, far more often than we do, that the Wliin has broken 

 completely through the beds. 



The Coal Measures of the Newcastle Coal-field have a thickness of 

 about 1800 or 2000 feet, the highest English Coal-measures not being 

 now represented in this district f . The Whin Sill, where at its 

 highest position in Mid Northumberland, lies below the base of the 

 Coal Measures, from 2000 to 3000 feet, according to the develop- 

 ment of the intervening beds. Therefore, if the intrusion took 

 place during the higher Coal-measure period, there would have 

 been a total thickness of from 3500 to 5000 feet of rock overlying 

 the AY hin. 



It may be a question whether the time which elapsed between 

 the deposition of the beds now found with the Whin, and the 

 pressure of some 5000 feet of superincumbent rock, would have 

 been sufficient to alter the lower beds from original sediment into 

 sandstone, shale, limestone, and ccal. If this be admitted, there is 

 still the question whether the pressure would have been sufficient to 

 keep the intruding sheet of lava between the lines of bedding. 



It is a fact of some moment, that when the "Whin lies high in the 

 series it seems most prone to break through the rocks ; but there 

 are many exceptions to this. 



If the intrusion took place when the whole country was deeply 

 covered by Secondary rocks, it is not likely that the pressure of from 

 1000 to 1500 feet of strata, more or less, would make any per- 

 ceptible difference. But if the intrusion took place during the 

 higher Coal- measure period, when the thickness of overlying rock 



* We do not here enter into the question of the relations of the Permian and 

 Carboniferous. In Northumberland and Durham the higher Coal Measures 

 are not present, and the Magnesian Limestone, with the underlying Yellow 

 Sand and Marl Slate, clearly lie unconformably upon such Coal Measures as 

 there occur. Stratigraphieally the break is there complete. 



t The highest Coal Measures of Northumberland are those just on the north 

 or downthrow side of the famous " 90-fathom dyke," near Gosforth and 

 Killing worth. The dyke, or fault, has here a throw of 200 fathoms. 



