Geology of High Teesdale. 189 
’ Assuming, that the trap rocks of High Teesdale are of volcanic 
origin, a question may arise respecting the time of their irruption 
among the strata. It is clear, that they took their present form 
posterior to the deposition of those members of the metalliferous 
formation with which they are in contact. It is also clear from 
what was stated in a former Paper (supra, p. 42.) that most of 
the dykes in the coal-field existed before the deposition of the 
magnesian limestone. Now, I think it perfectly certain, that the 
basaltic rocks of High Teesdale, were not formed after the 
dykes of the coal-field. It therefore follows, that these rocks must 
have existed in their present form before the deposition of the 
magnesian limestone. Thus, the time of their irruption, is limited 
to one great geological era. This era may, however, have been 
of long duration, and it by no means follows, that all the basaltic 
dykes of the coal-field are strictly contemporaneous; still less does 
it follow, that these basaltic dykes were formed at the same time 
with the imbedded masses of trap in High Teesdale (the Whin- 
Sill.) 
I think, from the facts stated in the previous Sections of this 
Paper, that the Whin-Sill cannot have been formed before the 
beds which rest immediately upon it; but it may, perhaps, have 
been formed at the same time with the trap in Lunedale, and 
with the dykes in Eglestone Burn, in which case, it must probably 
have been injected among the metalliferous strata, after the 
deposition of, at least, a part of the coal formation. 
If this last conclusion be admitted, we must allow, that the 
Whin-Sill has been produced by a lateral injection of volcanic 
matter in a state of igneous fusion. That such a mode of for- 
mation is possible, has been unequivocally demonstrated in Dr. 
Mac Culloch’s excellent Work on the Hebrides (See the Plates 
to the Geological Description of Sky). 
Our reluctance in admitting this explanation, arises from the 
Probable ‘Time 
of the Irruption. 
Lateral 
Injection. 
