PENGELLY — ON" 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 igneous 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 close 

 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 St. 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 seliorlaceous granite was first formed ; this was succeeded 

 by the porpliyritic variety when the first had become compact and 

 jointed; afterwards the elvans were formed and obtruded into the 

 * Geol. Trans., 2nd series, vol. v. part iii. p. OSG. 



