in Yorkshire and Durham. 213 
bounds of the coal-fields, and traverse indifferently all the 
newer strata which cross their line of direction. ‘The facts 
presented by the north coast of Ireland afford several iilustra- 
tions of the truth of this assertion. 
Mr. Winch, in the fourth volume of the Geological Trans- 
actions, has given many interesting details respecting the dykes* 
which intersect the great coal basin of Northumberland and 
Durham. They are in some instances filled with clay and 
rounded 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 ba- 
salt, 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 pene- 
trate 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 filling up the inequalities, or resting on the in- 
clined edges, of the coal measures. In all such cases, the frac- 
tures and contortions of the lower formation must have taken 
place prior to the deposition of the superincumbent horizontal 
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 hori- 
zontal beds which repose upon the disturbed strata. Such a 
rule as this may, however, admit of many exceptions. For 
no reason can be given @ priori, why the same forces, which 
produced the great fissures in our coal formations, should not 
again come into action in successive epochs in the natural hi- 
story of the earth. Accordingly, it is found that basaltic dykes 
are not confined to any particular set of strata, hut may occa- 
* In the North of England the term dyke is not confined to the descrip- 
tion of those fissures which have been filled with trap, but is extended to all 
the great faults and dislocations 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, 
sionally 
