256 | Prof, Sedgwick on some Trap Dykes 
state, that many dykes of similar origin wedge out before they 
reach the surface*. 
4. The dyke has once been in a fluid state. For it is 
moulded tovall the flexures of the chasm which it fills up. The 
same assertion is also proved by its crystalline texture. 
5. The materials of which it is composed are the same with 
those which abound in a great many varieties of recent lava. 
On this subject there is perhaps no difference of opinion. For 
the Wernerians at one time asserted, that recent lava was de- 
rived from the igneous fusion of trap rocks of aqueous origin. 
6. The effects produced by the dyke are such as might be 
expected from the intrusion of a great mass of ignited matter. 
'Phis assertion is fully established by the facts which have. been 
already stated. 
If, therefore, similar effects have originated in similar causes, 
we must conclude, that this-dyke, as well as all the other si- 
milar masses in the great Durham coal-field, are the undoubted 
monuments of ancient volcanic action. 
_ Conclusion. 
It is a matter of fact, which is independent of all theory, 
that an enormous mass of strata has been rent asunder; and 
it is probable that the rent has been prolonged to the ex- 
tent of fifty or sixty miles. If we exclude volcanic agency, 
what power in nature is there capable of producing such an 
effect? By supposing 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 exten- 
sive 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 cer- 
tain 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, 1823. 
P. S. Before this paper was sent to the press, I received 
two letters from my friend Mr. Wharton, of Oswald House, 
* See ProfessorHenslow’s paper on the Isle of Anglesea ; Dr. MacCulloch 
on the Hebrides, &c. &c. 
near 
