THE DARTMOOR VOLCANO. 
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constituents of granite, notably in the quartz, and arguing from 
these Mr. Sorby long since announced that the Cornish granites 
had as a mean consolidated at a temperature of 216 C, and under 
a pressure equivalent to that of 55,000 feet of rock. 2 The late 
Mr. Clifton Ward in the same way estimated the granites of 
Westmoreland and Cumberland to show a mean pressure of 
44,000 feet — the Skiddaw granite indicating 52,000, with a sug- 
gested temperature of 360 C. Mr. Sorby's mean for the Highland 
granites gives a pressure-equivalent of 76,000 feet. 
Mr. Sorby 's conclusions as to the origin of granite are that it 
might have been formed under conditions " combining at once 
igneous fusion, aqueous solution, and gaseous sublimation," and 
that the pressure under which granite solidified was analogous to 
that of lavas u solidified at the focus of their activity, as though 
these rocks were the non-erupted lavas of ancient volcanos variously 
protruded among the superincumbent strata." 
Professor Prestwich, adopting the hydro-thermal or extreme 
metamorphic view, criticises Mr. Sorby's details, while agreeing 
in the main with his inferences, and thinks that " granite must 
have consolidated under extreme pressure, and at a temperature 
probably not exceeding about 700 Fahrenheit, or under that of 
low red-heat." 
It may thus be accepted as a definite article of geological faith 
that granite was at one time in a highly heated and plastic state — 
to go no further. 
But it has been argued that granite rocks have been changed in 
situ — that the granite is simply the product of a gigantic stewing 
operation, on strata which practically occupy the same position 
now as before, though changed in character. 
The answer in our case is perfectly clear, whether on physical 
or on chemical grounds. If our granites are only altered forms of 
the rocks among which they lie, then one must pass into the 
other by gradual change. This is never the case. Wherever you 
trace the Devon or Cornish granites in contact with other rocks 
you find the line of junction perfectly sharp and distinct. Sir 
Warington Smyth has asserted "positively that careful research 
will reveal numerous places on these granite hills, to be seen 
sometimes with great distinctness underground . . . where the ends 
of schistose strata may be seen to abut against the face of granite." 
3 Quar. Jour. Geol. Soc. xiv. 
