418 W. TOrLEY AND G. A. LEBOUB, OX TTIE INTRUSIVE 



As a rule, we find that the thicker the Whin the coarser the grain 

 of the rock. This is perhaps due to the slower cooling of the thick 

 masses^allowing crystallization to develop more perfectly. 



The Whin Sill is not often vesicular, and still more rarely amyg- 

 daloidal. When it is, it is not more so near the upper surface than 

 elsewhere ; nor is it more so than some of the Whin Dykes which 

 traverse the country. No volcanic ash occurs in the district : nor is 

 there any spot which can be pointed out as certainly a vent or neck, 

 up which the trap came. 



6. Age and Origin. — The foregoing statements have proved that 

 the Whin Sill is newer than the beds of the Carboniferous -Lime- 

 stone Series in which it lies. But the question of the exact age of 

 the intrusive sheets yet remains to be determined. So far as Norths 

 umberland is concerned, the question must remain an open one. 



The Whin Sill is older than most of the faults of the district in 

 which it occurs, because these throw trap and sedimentary beds 

 alike. As already mentioned, we have no clear case of the Whin 

 being unaffected by (or later than) any of the faults. The Whin is 

 also clearly older than the mineral lodes of the district. Bnt what 

 the age of faults and Lodes may he we do not know. Some of the 

 faults are probably pre-Permian ; at least the Magnesiaa Limestone 

 appears to be unaffected by certain faults which are proved in Coal- 

 workings beneath*. Others, and these generally the large east-and- 

 west faults, are clearly post-Permian. Bnt we cannol generalize as 

 to the age of the fault.- in this district merely by their direction : and 

 even if we could, the known ii - of faults in the Whin Sill are 



not sufficiently numerous to allow us t<> apply the test with safety. 



In Scotland, Warwickshire, and Staffordshire there are intrusive 

 sheets of trap in the Coal Measures, hut none in the adjacent 

 Permians. Negative evidence in these cases strongly suggests that 

 the trap is pre-Permian; and the conclusion thus suggested is 

 generally adopted. Without wishing to call this conclusion in 

 question, we should like to make a few remarks upon this kind of 

 evidence. 



If the Permians can be shown to have been deposited upon the 

 denuded edges of tfo t op, the proof is complete. But it is only 

 very rarely that such proof can he given. Generally, the evidence 

 is of this nature :— Carboniferous rocks, in which intrusive sheets 

 of trap now occur, have been disturbed and denuded, and their edges 

 covered up by unconformable Permians. This, however, is not 

 sufficient proof ; or rather it is no proof at all ; for the trap may 

 have been intruded into the Carboniferous rocks long after the de- 

 position of the Permians. There is evidently a tendency for these 

 sheets of trap to keep along the lines of bedding; otherwise there would 

 never be any doubt as to their character. If the •intruding trap 

 began to force its way laterally first through the Carboniferous rocks, 

 it would probably, if possible, stay there. A difficulty here occurs 



* See Sheet 105 S.E. of the Geological-Survey Map, and the 6-inch Map of 

 the Durham Coal-field ; surveyed by Mr. Howell. 



