Trap dykes in 
the coal-fields, 
22 Professor Sepewick on Trap Dykes 
them is, that they are newer than the beds into which they 
have intruded. It is on this account that different observers have 
formed completely different views respecting the classification of 
certain formations of trap; each, im ambiguous cases, having 
adopted that opinion which happened to fall in with his favorite 
theory —In determining the origin of any one of these formations, 
it seems essential to inquire, (1) In what manner it is associated 
with other rocks. (2) What minerals enter into its composition. 
(3) What effects are produced by its presence. Satisfactory an- 
swers to these questions have been obtamed from so many quarters, 
that the discussions in which they have originated will perhaps 
soon terminate. It is my intention in this communication to 
bring together some facts, connected with the subject, which fell 
under my observation during last summer. 
Dykes and overlying masses of trap are of such ordinary oc- 
currence in many of our coal-fields, that they have sometimes 
been regarded as true members of the great coal formation. 
Should it, however, appear, that they have not originated in the 
same causes which formed those innumerable layers of sandstone, 
shale, ironstone, &c. which enter into the composition of the coal 
strata; but that they have been subsequently driven m among 
these beds by the irregular action of powerful disturbing forces ; 
we shall then be compelled to regard them, not as the subordinate 
members, but as the intrusive associates of the great coal forma- 
tion. In confirmation of this opinion it may be stated’; (1) That 
in many extensive coal-fields there are no traces of any beds or 
dykes of trap. (2) That in other places, such beds or dykes pass 
beyond the 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 illustrations 
of the truth of this assertion. 
Mr. Winch, in the fourth volume of the Geological’ Trans- 
actions, has given many interesting details respecting the 
