212 Prof. Sedgwick on,some Trap Dykes 
newer strata, and distinctive characters have been pointed out 
by which it has been attempted to separate the several forma- 
tions from each other. As observations have become more 
widely extended, many of the conclusions founded on such 
characters have proved to be fallacious ; and it isnow generally 
admitted, that the mineralogical composition of any system of 
trap rocks gives us little information respecting its antiquity or 
probable‘associations. When strata rest conformably upon each 
other, in such a way as to indicate a‘continued succession of de- 
positions, we can immediately determine, at least, their rela- 
tive antiquity, and may often adopt some natural or artificial 
arrangement which will greatly facilitate their description. 
But formations, which appear as dykes and overlying masses, 
afford no such facilities for correct classification; and the only 
general conclusion which we can arrive at respecting them is, 
that they are newer than the beds into which they have in- 
truded. It is on this account that different observers have 
formed completely different views respecting the classification 
of certain formations of trap ; each, in ambiguous cases, having 
adopted that opinion which happened to fall in with his fa- 
vourite 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 answers to these questions have been 
obtained 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. 
Trap Dykes in the Coal-fields. 
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. 
Shouldut, 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 composi- 
tion'of the coal strata; but that they have been subsequently 
driven in among these beds by the irregular action of power- 
ful disturbing forces; we’ shall’ then be compelled to regard 
them, not as the subordinate. members, but as the intrusive 
associates of the great coal formation, 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 
