III. On the Phenomena connected with some Trap Dykes 
in Yorkshire and Durham. 
By tHe Rev. ADAM SEDGWICK, M.A. F.R.S. M.G.S 
FELLOW OF TRINITY COLLEGE, AND WOODWARDIAN PROFESSOR IN THE 
UNIVERSITY OF CAMBRIDGE, 
[Read May 20, 1822.] 
Tue various phenomena presented by trap rocks have long jyoduction. 
engaged the attention of Geologists. Different ages have been 
assigned to them, founded on their union with older or newer 
strata, and distinctive characters have been pointed out by which 
it has been attempted to separate the several formations 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 is now generally admitted, that the mineralogical 
composition of any system of trap rocks gives us little informa- 
tion respecting its antiquity or probable associations. When 
strata rest conformably upon each other, in such a way as to 
indicate a continued succession of depositions, we can immediately 
determine, at least, their relative 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 
