in Yorkshire and Durham. 23 
dykes*. which intersect the great. coal basin of Northumber- 
land and Durham. They are. in some instances filled with 
clay and. roGnded pebbles or shattered fragments of sandstone, 
mixed with other materials derived from the neighbouring 
rocks, and their whole appearance plainly indicates the violent 
nature of the forces by which the solid strata have been cleft 
asunder... In other instances, the fissures are filled with a variety 
of basalt, which rises like a great partition wall through all the 
beds of the formation. .(Geol. Trans. vol. IV. p.21—30.) It is 
the opinion of Mr. Winch that these basaltic dykes never pass 
up into the magnesian limestone which reposes immediately on 
the coal strata. Thus, for example, the cliff of Tynemouth castle 
is intersected by a basaltic dyke which does not penetrate the 
capping of magnesian limestone. 
Every one who is acquainted with the details of English 
Geology. must have remarked, that our newer strata, down 
to the magnesian limestone inclusive, are generally unconform- 
able to, all the older rocks. Thus in numberless instances, 
more especially in the West of England, we find some of 
the newer strata fillmg up the imequalities, or resting on the 
inclined edges, of the coal measures. In all such cases, the 
fractures and contortions of the lower formation must have 
taken place prior to the deposition of the supermcumbent hori- 
zontal beds. Now if it appear, that masses of trap are not only 
the common associates of such fractures and dislocations, but 
sometimes the very instruments by which they have been pro- 
duced ; it follows, almost of necessity, that the dykes we have 
been describing will not generally be found among the horizontal 
beds which repose upon the disturbed strata. Such a‘rule as this 
* In the North of England the term dyke is not confined to the description of those 
fissures which have been filled with trap, but is extended to all the great faults and disloca- 
tions which intersect the strata in a nearly vertical direction. A want of attention to this 
extended use of the word has given rise to occasional mis-statements and false inferences. 
