212 J.J. H. TEALL—PETROLOGICAL NOTES ON 
to Armathwaite, where it crosses the Eden at the weir about half a 
mile above the bridge. There are many good exposures in the 
neighbourhood of Armathwaite. Its thickness in the railway-cutting 
is about 18 yards. 
It thus appears that a dyke having characters which are constant 
and at the same time different from those of any other igneous rock 
known to me in the north of Kngland may be traced at intervals 
from Maybecks in the east to Armathwaite in the west, a distance of 
about 90 miles ; that it keeps on the whole a general W.N.W. direc- 
tion but is liable here and there to lateral shifts and local deviations : 
and that it may be seen in one place, and proved by colliery work- 
ings in others, to die out before reaching the surface; so that in 
districts where it is not actually seen it may reasonably be supposed 
to exist underground. 
Doubts have been expressed as to the continuity of the Cockfield — 
and Cleveland dykes. When, however, we see that the petrological 
characters of these two dykes are not only alike, as has been re- 
marked by Prof. Sedgwick and others, but also peculiar, a point which 
is, I believe, now insisted upon for the first time, there can, I think, 
be no longer any hesitation in admitting their continuity, especially as 
they follow each other very much in the samestraightline. And if 
we admit the continuity of the Cleveland and Cockfield dykes, we are 
compelled by the same kind of evidence to regard the Armathwaite 
dyke as a portion of the same igneous outburst. . 
What is the age of this dyke? The latest formations intersected 
and altered by it are the oolitic sandstones of the Yorkshire moors. 
It must therefore be post-Jurassic. The only period subsequent to 
the Jurassic in which volcanic action is known to have taken place 
in Western Europe is the Tertiary, and especially the Miocene; 
and inasmuch as the dyke in question points by its direction to the 
north of Ireland, and west of Scotland, districts celebrated for the 
enormous development of Miocene volcanic rocks, we seem com- 
pelled to regard it as of Miocene age. 
It has been suggested by some that there may be a connexion 
between the Great Whinsill of Teesdale and the dyke in question. 
This seems to me in the highest degree improbable. There are 
marked chemical and physical differences between the two rocks, 
even where, as at Tyne Head, they occur in close proximity, and 
where the Cockfield Dyke has in all probability cut through the 
Whinsill. The Whinsill is composed of a rock of more basic com- 
position and higher specific gravity than that of the dyke. It is 
moreover holocrystalline, whereas the dyke contains a considerable 
amount of imperfectly individualized matter. It is interesting to 
note that the Whinsill is similar in composition, structure, and mode 
of occurrence to the diabase (? dolerite) which is so extensively 
developed in the Triassico-Jurassic strata of Nova Scotia, Massa- 
chusetts, Connecticut, New Jersey*, Pennsylvania,and North Carolina, 
and which possesses throughout the whole of its enormous range, 
* See E. 8. Dana, ‘American Journal of Science,’ 1874, ser. 3, vol. viii. p. 890, 
and G. W. Hawes inthe same Journal for 1875, vol. ix. p. 185. 
