70 MR, E. N. WOKTH ON THE IGNEOUS CONSTITUENTS OF THE 



had been furnished hy the inferior rocks of its immediate neighbour- 

 hood." While, however, Mr. Conybeare names granite, none of the 

 descriptions given by him apply to genuine granitic rocks. 



Sir H. de la Beche, in his vrell-known ' Eeport on the Geology of 

 (yornwall, Devon, and West Somerset,' indicates the common local 

 origin of these clastic beds, but notes the conglomerates of the strip 

 of Trias trending west from Crediton to Jacobstow as comprising 

 " besides rounded fragments of the adjoining carbonaceous sand- 

 stones and slates, pebbles of porphyritic rocks, and some which 

 seem rolled fragments of Dartmoor granite " ; and the conglo- 

 merates " from the neighbourhood of Exeter to the sea" as made up 

 *' of angular fragments and rolled portions of the various sandstones, 

 slates, and limestones on which they rest," with certain igneous 

 rocks not so identified — most numerous towards the Teign ; and 

 finally he thus sums up the evidence : — 



" When the detritus composing the conglomerate occurs in suffi- 

 ciently large pieces there is little difficulty in perceiving that with 

 the exception of some fragments of igneous rocks in South Devon, it 

 is derived from the adjacent and disturbed older beds, or from 

 igneous rocks associated with the lower part of the series ; and we 

 infer that the finer portions are more comminuted parts of the same 

 rocks." 



Half a century since, therefore, the only question treated as open, 

 touching the constituents of these conglomeratic beds, was the origin 

 of certain of their igneous components, and that has remained the 

 single point of doubt or controversy, though it may seem strange 

 that the invariable local character of the sedimentary constituents 

 had not almost forced the conclusion that their igneous associates 

 were local too. 



Sir Henry de la Beche failed to detect any granite similar to 

 that of Dartmoor in the conglomerates south of Exeter to the sea, 

 *' while fragments of other igneous rocks, chiefly porphyries, a large 

 proportion of which are not known to occur in place, are extremely 

 abundant." Eis general suggestion with regard to these porphyries 

 was that they might " readily have formed portions of igneous 

 masses covered up by the Eed Sandstone series, and it is by no 

 means improbable that such masses are thus concealed "*. 



While Sir Henry had thus accepted the occurrence of rocks 

 resembling " some varieties of Dartmoor Granite " in the conglo- 

 merates near jSTorth Tawton and Sampford Courtenay, Mr. Godwin- 

 Austen, E.E,.S., positively denied in the most absolute terms the 

 presence of any granite among the materials of the red conglomerates 

 of Devon. " IN'o granite pebbles have been found among the 

 various materials of which the new conglomerates are composed "t- 

 It is no doubt partly due to the uncertainty of one authority and 

 the overcertainty of the other, that this question has remained so 

 long in suspense. But the chief hindrance may have been the 



* The citations from Sir Henry de la Be "^he are taken from the chapter in his 

 Report on the "Red Sandstone Series," _2;as£"»«, pp. 193-220. 

 t Geol. Trans. 2 ser. vol. vi. p. 478. 



