TRIASSTC BRKCCIAS AND CONGLOMERATES OF SOUTH DEVON. 71 



search for Dartmoor granite in particular, instead of Dartmoor rocks 

 in general. 



The presence of indubitable fragments of Dartmoor granite in 

 the conglomerates of North Tawton and Haldon has, however, long 

 been recognized. They were found there by Mr. Yicary, F.G.S., 

 and somewhat later bj Mr. Pengelly, F.It.S., in company with him, 

 at least thirty years since. 



And so Mr. G. W. Ormerod, F.G.S., in a paper read before this 

 Society, on " The Murchisonite beds of the Valley of the Exe," in 

 1875 *, identified the Murchisonite as clearly of Dartmoor origin, 

 and mentioned the occurrence of granite pebbles at Teignmouth. 



But it is to Mr. Yicary that we owe the most definite pronounce- 

 ment on this head. In a paper read before the Devonshire Associa- 

 tion for the Advancement of Science, Literature, and Art in 1867 t, 

 he avers unhesitatingly that " the masses containing Murchisonite 

 are but altered portions of the granite of Dartmoor," the variation 

 in colour and texture between the Murchisonite pebbles and the 

 in situ granite being quite paralleled by the changes which have 

 taken place in fragments of Carboniferous Grit and Devonian Lime- 

 stone in the same association. Moreover, he remarks that '' they 

 occur in the Trias in close proximity with pebbles of schorl and 

 altered rock, a collocation strikingly similar to that met with in 

 Dartmoor streams." His final conclusion is (and I have his authority 

 for saying that it is only strengthened by subsequent investigations), 

 " If the granite was exposed at the surface at the era of the red 

 rocks, Dartmoor must have furnished a large proportion of the 

 conglomerated materials. I feel confident that a thorough exami- 

 nation of these materials will prove that granite is a far more 

 important constituent than geologists generally suppose." 



More recently it has been suggested that we should look else- 

 where especially for the origin of the characteristic red felspar 

 porphyries ; and Mr. W. A. E. Ussher, E.G.S., has suggested that 

 their source may be in the Channel area J. 



Mr. Ussher also points out that while the breccias or con- 

 glomerates frequently contain igneous fragments distinctly referable 

 to the destruction of such igneous patches as those of Washfield, 

 Killerton, Silverton, and Spencecombe, " the porphyritic and Murchi- 

 sonite fragments are confined to the South-Devon breccia." 



Instead of suggesting that these comparatively unworn fragments 

 are in any sense "foreign," such facts seem to mo to assign them a 

 distinctly local character, and to indicate a probable origin in the 

 direction towards which the area of the beds containing them nar- 

 rows. We have another valuable hint in the fact that these frag- 

 ments attain their largest size in the vicinity of Teignmouth, and 

 that thence both to the north and south they rapidly graduate 

 downward to very moderate dimensions, and, in fact, into sand. It 



* Quart. Journ. Geol. Soc. vol. xxxi. pp. 346-354. 

 t Trans, Dev. Assoc, vol. ii. pp. 200-202. 



PP- 

 Vide Quart. Journ. Geol. Soc. vol. xxxiv. pp. 461-402, vol. xxw. pp. 245- 



2()7. 



