General sum- 
mary. 
38 Professor Sepawick on Trap Dykes 
causes: so that the igneous origin of a large class of trap dykes 
seems to be established by evidence which is almost irresistible. 
It is urged to no purpose, that Lydian stone and glance-coal 
occur in places which have never been influenced by volcanic 
action. The assertion may be true, but is of no value in deter- 
mining the question; unless it can be shewn, that substances, 
similar to those derived from the sides of the dykes, are found 
in other parts of the same district which are removed from their 
influence. This however is not the case, for the enormous ex- 
cavations which have been carried on in the great coal-basin of 
Northumberland and Durham have, with one ambiguous excep- 
tion*, made us acquainted with no similar substances excepting 
those which appear to have been produced by similar agents. 
It may be proper briefly to enumerate some of the facts 
which are established by a detailed examination of the great 
dyke, and which will, perhaps, be considered to place its origin 
out of all doubt. 
(1) It is more recent than the formations which it traverses. 
For it occupies the interval between beds which were evidently 
once continuous; but have been subsequently broken up and 
severed by some great convulsion. 
(2) It was consolidated prior to the last great catastrophe 
which formed the beds of superficial gravel, and excavated the 
secondary vallies. In proof of this we need only state, that it 
partakes of all the inequalities of the districts through which it 
passes, rising with the hills and falling with the vallies, so that 
*See the Geological Transactions, vol. IV. p. 27. The case is obviously ambiguous, 
because the effect of a large mass of trap on a bed of coal may be propagated to a considerable 
distance. The very change described by Mr. Winch may therefore have been effected by 
a mass of trap which is not exposed in the workings. We must carefully distinguish between 
the phenomena here described, and the effects of those dislocations which so commonly inter- 
sect the coal strata. In these latter instances the coal beds are often deteriorated on both 
sides of the line of fault by the mere mechanical effects of the rupture. 
