PENGELLY — 0^' THE AGE OF THE DARTMOOR GRANITES. 19 
Whatever may be our opinion respecting the origin of granite, — 
whether we hold it to be a strictly igueous or a thermo-aqueous pro- 
duct, an original or a superimposed phase of rock existence, — we 
are probably all agreed that it was formed in plutonic depths, a 
hypogene formation requiring for its elaboration enormous pressure, 
and therefore at least commensurate resistance in a superincumbent 
crust. In the case before us, the overlying mass existed at the cloise 
of the Carboniferous period, or the granitic form could not have been 
assumed by the Dartmoor rocks ; and it must have been removed 
and the granites laid bare before the conglomerate era, or fragments 
of the latter could never have found their way to Haldon. 
Mr. Sorby estimates the pressure under which the St. Austell 
granite was formed as equivalent to that of 32,400 feet rock ; that of 
the mean of the Cornish granite at 50,000 feet ; and that of Ding- 
dong Mine, near Penzance, at 63,000 feet. He gives no estimate 
for Dartmoor, but taking his lowest, the ^t. Austell figures, we have a 
pressure equivalent to that of a pile of rock six miles in thickness ; 
but, since the pressure was probably due to the expanding power of 
some agency acting beneath or within the granitized mass, — requiring 
resistance and not pressure, strength and not weight in the overlying 
crust, — we will content ourselves with a small fraction of this : never- 
theless there must have been a solid crust of vast thickness for de- 
nudation to strip off before a granite pebble could have travelled 
to Haldon. Even if we suppose that some paroxysm uplifted the 
granite in a solid state, so as to shiver the overlying masses, and 
thereby facilitate the work of denudation, still the removal of such a 
mass of rock must have required an amourt of time so vast, that it 
seems totally impossible to regard the red conglomerates and sand- 
stones as more ancient than the Lower Trias ; and, even thus, what 
an incalculably great value does this stamp on the units of geological 
chronology ! 
The supposition, however, that the granite was thus thrust through 
the overlying rocks is altogether improbable, for the latter appear 
to have shared in all the great movements which the former may 
have undergone. According to Sedgwick and Murchison, the granite 
veins in the older surrounding rocks, " taken in general, are mere 
prolongations of the central granite, inseparable from it, and contem- 
poraneous with it."* 
The time of denudation, moreover, vast as it probably was, 
formed but a fraction of the period separating the culmiferous and 
red rocks. At the close of the Carboniferous period there was no 
Dartmoor granite ; after this we have, according to Mr. Godwin- 
Austen, the formation of three distinct masses of granitoid rocks, 
very distinguishable from one another, clearly results of dissimilar 
conditions within the same area, and therefore referable to different 
times. The 5c7ior/«ceow5 granite was jfirst formed; this was succeeded 
by the porpTiyritic variety when the first had become compact and 
jointed; afterwards the elvans were formed and obtruded into the 
* Geol. Trans., 2iid series, vol. v. part iii. p. 686. 
