12 



THE GEOLOGIST. 



It has long been known that the age of the granitic rocks of Dart- 

 moor can be safely limited on the side of antiquity. That they are 

 less ancient than the culmiferous beds of North and Central Devon 

 has been established on satisfactory evidence produced by various 

 observers. 



Sir Henry De la Beche, in his ' Eeport,' says, " The intrusion of 

 the Dartmoor mass was certainly after the deposit of the carbo- 

 naceous series of North Devon, be the age of that series what it may ; 

 it thrusts the southern portion of this series northwards to Oak- 

 hampton, cuts off the ends of trappean bands and of associated beds 

 of grit and shale near Cristow and Bridford, and sends veins into it 

 in the valley of the Dart, at the junction of the two masses of rock." * 



Professor Sedgwick and Sir B,. I. Murchison, in their paper on 

 " The Physical Structure and Older Stratified Deposits of Devon- 

 shire," say, " Granite veins, passing from the central mass into the 

 superimposed stratified rocks, are found on all sides of Dartmoor. 

 We have seen them above Ivybridge, injected amongst the oldest 

 slates of Devonshire ; and near Oakhampton we have seen them in 

 like manner, penetrating the culm-measures ; and they are finely ex- 

 posed in the beautiful gorges of the Teign and the Dart, where those 

 rivers descend from the granite to the culmiferous series. These ex- 

 amples, to which we could add many more, are sufficient for our pur- 

 pose. Now these veins, taken in general, are mere prolongations of 

 the central granite, inseparable from it, and contemporaneous with 

 it ; they cannot therefore (as the granite is one mass) be contempo- 

 raneous with stratified rocks of different ages. Consequently they 

 are true veins of injection, and the granite was protruded at a time 

 posterior to all the other stratified systems."! In another part of 

 the same paper these authors go on to say, " It appears that the rocks 

 of Devon and Cornwall belong to three periods of formation. The 

 oldest includes the various groups of slate rocks, and at least a part 

 of the associated, traps. The next includes the culm series, the upper 

 division of which contains fossils identical with those in the upper 

 division of the coal measures. The granite belongs to the newest 

 period." J 



Mr. Godwin- Austen, in his paper on " The Geology of the South- 

 East of Devonshire," speaks of similar granite veins, and, in a some- 

 what qualified manner, confirms the opinion of the authors just quoted 

 respecting the chronology of the granite ; stating that " The observa- 

 tion of Professor Sedgwick and Mr. Murchison, above quoted, as to 

 the age of the Dartmoor granite, applies necessarily to the schorly 

 portion alone, which comes in contact with sedimentary deposits, for 

 the entire mass is not of the same age." He then gives a figure of a 

 section, " showing the manner in which the usual porphyritic granite 

 has intruded itself among such as had already become compact and 

 jointed, and containing schorl," and adds, " Again, this porphyritic 



* ' Report on the Geology of Cornwall, Devon, etc.,' p. 165. 

 t Geol. Trans., 2nd series, vol. v. part iii. p. GSG. 

 X Geol. Trans., 2nd series, vol. v. part iii. p. 687. 



