40 Professor Sepaewick on Trap Dykes 
posing such phenomena the effects of volcanic action, we bring 
into operation no causes but those which are known to exist, 
and are adequate to effects even more extensive than those which 
have been described. 
Combining this observation with the facts described with 
minute detail in the preceding parts of this paper, we obtain 
a chain of evidence, in favour of the igneous origin of a certain 
class of trap dykes, not one link of which appears to be defective. 
It is not to be denied, that the associations of trap rocks may 
in other cases present great difficulties to the igneous theorist. 
But these difficulties are not the present subject of consideration. 
I have confined myself, as far as possible, to a statement of 
facts, and I have only attempted to record such conclusions as 
a review of those facts appeared fully to justify. 
Trin. Coll. March 12, 1893. 
P.S. Before this paper was sent to the press, I received two 
letters from my friend Mr. Wharton, of Oswald House, near 
Durham, communicating some very interesting facts connected 
with the appearance of a basaltic dyke; which ranges from the 
escarpment of the magnesian limestone (at Quarrington Hill, 
a few miles to the east of Durham) through the great coal-field, 
in a direction about W.S.W. It is found along this line at 
Crowtrees, Tarsdale, Hett, Tudhoe, Whitworth, and Constantine 
farm. From the last mentioned place, it passes along the same 
line of bearing, through the collieries of Bitchburn and Hargill 
Hill, to a spot near the confluence of Bedburn Beck and the 
river Wear, where it is well exposed on the surface of the ground; 
and it is known to pass up the Bedburn Beck valley towards 
Egglestone Moor. If prolonged a few miles in the same direction, 
