512 Alecc Somervail — Origin of Dartmoor Granite. 



and De la Beche,^ as was also the fact of the conglomerates 

 containing fragments of the cherts and other Lower Culm rocks. 

 As to the age of these conglomerates, the author thinks they might, 

 for good reasons, be referred to the Pennant Grit series of the South 

 Wales and Bristol Coalfields, both of which, curiously enough, contain 

 boulders or pebbles of coal and anthracite from the lower measures, 

 indicating similar conditions to what existed in South Devon. 



The question of the time required during this interval or break in 

 the sequence between the Lower and Upper Culm deposits, for the 

 protrusion of the granite, is now sufficiently well disposed of. So 

 also is the question of the protrusion of the granite following the 

 old and weakened line of former Culm basic products. There is, 

 moreover, much good reason to suppose that the acid magma of 

 the granite, and its perhaps more trachyte-like external portions, 

 followed very hard after the great basic masses already mentioned. 



The most important question of all, however, remains to be 

 answered — Do the contents of the conglomerates contain any 

 materials such as might be derived from the waste of granitic or 

 trachyte-like rocks ? To the eye, and with the aid of a lens, 

 these conglomerates, besides the chert fragments, contain much 

 arkose materials, such as quartz, felspars, etc. ; and mica in large 

 flakes are most abundant in them. These granitic and trachyte- 

 like materials are present in such large quantities as would make 

 it very difficult to find their source of origin from the wear and tear 

 of any other of the ordinary Devonian or Lower Culm rocks. The 

 felspar crystals and particles are so abundant, so large, and so 

 distinct that the author deemed it quite unnecessary to have 

 specimens submitted for microscopical examination. Fragments of 

 a vai'iety of granite are also present. 



With regai'd to the alleged fragments of the Dartmoor granite said 

 to have been found in the adjoining Permian breccias by the late 

 Mr. Pengelly ^ and others, the writer would remark that if this be 

 really so it would accord very much better with the inter- or late 

 Culm age of that rock than with its Permian or Pre-Triassic age, as 

 formerly held. 



In the bi'eccias referred to the author has been able to detect many 

 examples and varieties of a kind of quartz-porphyry. These latter 

 might in some way or other be connected with the outer granite 

 mass of Dartmoor, as also might be the large and numerous crystals 

 of Murchisonite found in the breccias, which seem to have no 

 connection with the subsequent basic flows in that formation. 



It is rather unfortunate that none of these Upper Culm rocks or 

 conglomerates are found in connection with the other bosses of 

 granite running into Cornwall, whereby we might better test their 

 age ; but many reasons combine to show that they all belong to the 

 same period of protrusion. 



There are many other points, such as the denudation of the Devonian 



^ Trans. Geol. Soc, ser. ii, vol. vi, p. 457 ; Geol. Rep. of Cornwall, Devon, etc., 

 p. 111. 



2 Eep. Brit. Assoc, 1861, p. 127. 



