26 
UPHEAVING OF “ IGNEOUS ROCKS.” 
ance of displacement in the latter, or at most with 
the occurrence of a fault , or slight disturbance at 
their brinks; but, on the other hand, there are cases 
where mechanical and chemical combination of 
connected rocks, igneous on the one hand, and 
transition on the other, sufficiently points out, if not 
disturbance, an extensive igneous action occurring 
at the period of elevation, and probably indeed, 
while at some depth below the surface. Our mica- 
slate and ribbon jasper are instances of these effects, 
the former may originally have been ordinary schist 
reposing on granite at some depth, and thrust up 
at the time of the granitic elevation, and converted 
simultaneously into its present structure ; ribbon 
jasper again, may have, in its former state, been 
ordinary slate, and converted during the rising of 
the trappean mass into the substance now spoken 
of. Mr. Prideaux observes that several hills round 
the moor, are constituted of schorl, quartz and slate 
in a state of aggregation.* Where granite runs in 
narrow veins through slate, it is reasonable to infer 
that this combination took place in a subterranean 
situation, and that the whole was then upheaved, 
but, it does not necessarily involve the conclusion 
that both rocks are of one age with regard to original 
formation, on the contrary, I have elsewhere stated 
my conviction, from the evident connexion of all our 
varieties of slate, that they (the slates) are of con¬ 
temporaneous deposition. 
But, if slates, and other rocks, never supposed to 
have been subjected to igneous action, are among 
themselves conjoined, and reciprocally blended, it 
might be enquired, whether the cause of mechanical 
and chemical blending on the one hand, were not 
the same as on the other. The method by which a 
* Geological Survey of the country round Plymouth, by John 
Prideaux, Esq.— Trans. Plymouth Inst. 
