PENGELLY — ON THE AGE OF THE DAltTMOOR GRANITES. 13 



and micaceous granite is traversed by elvans of a compact, fine-grained 

 stone, presenting no distinct crystallization of any of its constituents, 

 and they have evidently been protruded posterior to the consolida- 

 tion of the rocks in which they occur. The facts here noticed war- 

 rant the conclusion that it (the Dartmoor region) contains granite of 

 three distinct ages." * 



More recently Mr. Ormerod, who most assiduously uses the faci- 

 lities which his residence at Chagford gives him for the study of this 

 subject, has mentioned several localities where granite veins occur in 

 the carbonaceous rocks ; he states that, at one place on the river 

 Teign, " the veins throw off branches into the adjoining rock, and 

 vary in thickness from a thin filament to a breadth of about eighteen 

 feet." He adds, that "the veins contain portions from the adjoining 

 carbonaceous beds, sometimes so slightly removed from the original 

 position that it can be traced ; in the larger veins some of the masses 

 are rounded, as if they had undergone attrition, but some (about a 

 cubic foot in size) still preserve their angularity." f 



There can be no doubt, then, that the Dartmoor granites are less 

 ancient than the culmiferous beds of North and Central Devon. 

 Our next business is to find, if possible, a modern limit to their age. 



Amongst the stratified rocks of the county, the red conglomerates 

 and sandstones, which give such a character to the cliffs and soil of 

 South Devon, succeed, in ascending order, the culmiferous beds 

 already spoken of ; they are the next more modern. Now con- 

 glomerates may be regarded as natural museums, in which we are 

 likely to find specimens of all pre-existing rocks occurring in their 

 neighbourhood, and the fact that any rock existing in a given loca- 

 lity has no representative fragment in an adjacent conglomerate, though 

 merely negative evidence, would not be a bad, though by no means 

 an unimpeachable, basis on which to found the opinion that such 

 rock is more modern than the conglomerate thus destitute of any 

 indication of its existence. Such an opinion, however, would, of 

 course, be overthrown by the first fragment which further research 

 might bring to light. 



Sir Henry De la Beche says, " The evidence of the Dartmoor 

 granite having occupied its present relative position, anterior to the 

 early part of the (New) Red Sandstone, is not always so clear as could 

 be desired; for among all the pebbles of the red conglomerate ex- 

 tending from Torbay to Exeter, we have not been able to detect any 

 portion of it, though the granite ranges so near that part of the red 

 conglomerate. In the tongue of red sandstone and conglomerate 

 which runs from Crediton, amid the carbonaceous series by North 

 Tawton and Sampford Courtney to Jacobstow, we have, however, 

 detected pebbles like some varieties of Dartmoor granite." J 



It must be confessed that this is not a very pronounced opinion in 

 favour of the North Tawton pebbles being of Dartmoor origin. In 



* Geol. Trans., 2nd series, vol. vi. part ii. p. 477- 



f Quart. Jour. Geoi. Soc, vol \v. p. 192. 



| ' Report on tlic Geology of Cornwall, Devon, etc./ p. 1C6. 



