258 Prof. Sedgwick on Trap Dykes in Yorkshire and Durham. 
5. The dyke appears to decrease in width ‘as it rises to- 
wards the surface. ‘Thus, in Crowtrees colliery, the width of 
the dyke, where it is cut through at the depth of fifteen fa- 
thoms, is nearly twice as great as at the surface. 
6. It does not appear at Quarrington Hill to cut through 
a bed of sand and pebbles, which lies between the highest beds 
of the coal-formation and the magnesian limestone. 
The importance of these facts in confirming the theoretical 
views given in the preceding paper, is too obvious to need any 
explanation. 
Mr. Winch asserts (Geological Transactions, vol. iv. p.25),_ 
‘¢ that he has never been able to trace any of these basaltic 
veins into the magnesian limestone, and is almost certain that, 
with other members of the coal-formation, they are covered by 
it.” The dyke just described affords some additional evidence 
in support of this opinion. Moreover, it appears, in its ge- 
neral relations, to agree so exactly with the Cockfield Fell 
dyke, that I now cannot help suspecting, that this latter also 
belongs to the class of * basaltic veins” which do not pass up 
into the magnesian limestone, though I inclined to a different 
opinion when the preceding paper was written. 
Respecting the prolongation of the Cockfield Fell dyke 
through the region of the magnesian limestone, there are con- 
flicting probabilities which lead to directly opposite conclu- 
sions. ‘The near agreement in the direction and dip of the 
Cockfield Fell and Cleveland dykes, has generally been sup- 
posed to afford sufficient evidence for their continuity. If this 
opinion be adopted, we must, I think, be compelled to admit 
the existence of a dyke through all the intermediate district*. 
On the contrary, there is no direct evidence for the existence 
of any trap associated with the magnesian limestone; and the 
relations of all the analogous formations in the coal district 
seem to prove, that the Cockfield Fell dyke cannot pass out 
of the limits of the coal-formation. 
If we adopt this latter opinion, we must admit that the dykes 
of Cockfield Fell and Cleveland (notwithstanding the agree- 
ment in their line of direction) belong to two distinct epochs. 
After all, the question is only one of local interest; and, as far 
as regards the leading object of this paper, of no importance 
whatsoever. 
Through the kindness of T. R. Underwood, Esq. of Paris, 
I have become acquainted with the results of an examination 
of specimens from several English trap dykes by Professor 
Cordier. I will subjoin his description of such specimens as 
were derived from localities alluded to in the preceding paper. 
* See the observations at p. 217 of this paper. 
“ No. 1. 
a 
