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Geology of High Teesdale. 191 
the weight of the incumbent beds, an upheaving motion must 
take place, some of the solid masses will become broken or dis- 
jointed, the lava will be propelled between them, and the lateral 
pressure will produce a lateral injection. Neither can I see 
any assignable limit to the extent of such an injection, as long 
as the superior beds remain unbroken, and the elastic forces are 
in undiminished action. If, however, the continuity of the upper 
beds were once broken, the melted lava would instantly occupy 
the fissure with a velocity proportioned to the pressure, the elastic 
fluids would find a new vent, and the horizontal motion of the 
lava would cease altogether. 
If voleanic forces ever have acted on a great scale upon un- 
broken and nearly horizontal strata, especially while such strata 
were under the pressure of the sea, the formation of tabular and 
vertical masses of lava (nearly resembling the Whin-Sill and dykes 
described in this Paper) appears to me to be a natural consequence 
of such action. Where the pressure of the sea is removed, and 
the crust of the earth is broken through, volcanic fluids find 
a ready escape, eruptions of lava are confined to one spot, and 
the operations are of a class altogether distinct from those which 
produced the phenomena of High Teesdale. Though the pre- 
ceding statements are purely hypothetical, yet, if they shew the 
possibility of explaining the appearances in Teesdale by any modi- 
fication of voleanic action, they will answer the purpose intended. 
For, after the facts adduced in the preceding Sections of this Paper, 
there cannot, I think, be any reasonable doubt that the trap 
rocks of that region are of igneous origin. 
The earth’s surface has been modified by so many disturbing 
forces, since the deposition of the great carboniferous formation, 
and the whole face of nature has been so much changed by the 
last great catastrophe which formed the superficial gravel, and 
Obscurity in 
Geological 
Questions. 
