Alex 8omervail — Origin of Dartmoor Granite. 511 



both in Devonian and in Lower Culm times. In the vicinity of 

 the other granitic bosses volcanic action was also active throughout 

 Devonian times, and continued for long periods in these and other 

 parts of the counties of Cornwall and Devon to produce highly basic 

 products ; culminating during the Lower Culm period in Devonshire 

 in those very extensive basic products extending in a line through 

 what is now Central Dartmoor. 



These long-continued, highly basic products, however, at length 

 came to a close, possibly from sheer exhaustion, and seem to have 

 been followed by the highly acid products which now form the 

 granites of Dartmoor and Cornwall. 



Some of the views suggested in this pa"per occurred to the writer 

 when in Central France, sitting on the basic products of the Puy de 

 Parion, gazing at the trachyte mass of the Puy de Dome. That 

 enormous mountain of domite, as it is termed, with others in the 

 same chain of puys, together with masses of trachyte elsewhere, 

 if one could exactly explain their history, might throw much light 

 on the granite bosses of Dartmoor. In the case of the Puy de 

 Dome and other trachyte puys, there are good reasons for regarding 

 these acid protrusions as later than the basic ones, as there are also 

 for the acid magma which now forms our own Dartmoor granite. 

 The Dartmoor granite, though now essentially a portion of a once 

 deeply-seated core, was doubtless formerly represented in its upper 

 and outer portions by a variety of materials — necessarily arising from 

 loss of heat, pressure, and more rapid cooling — of a more trachytitic 

 nature. The boss of granite as it now remains has had stripped from 

 it its more external parts. This even seems true of the Puy de 

 Dome itself, and other trachyte masses like those of the Rhine 

 district, which certainly have suffered a considerable amount of 

 denudation within the very limited period since their formation. 



The question might now be asked — Is the interval between the 

 Lower Culm and the overlying conglomerates, with the included 

 fragments of the former, sufficiently great to allow for the formation 

 or protrusion of the granite ? The author is firmly convinced 

 that it is, for the reasons already given. Messrs. Hinde & Fox, 

 in their paper already referred to, say : " It is hardly probable 

 that the Radiolarian beds are directly succeeded by beds of 

 coarse clastic materials, i.e. the Ugbrooke Park conglomerates." 

 The author's own personal observation of these conglomerates 

 impresses him with the fact of their being widely separated in time 

 from the ehert series. In addition to the reasons already mentioned, 

 the conglomerate series never seem to occur in direct succession 

 above the lower members of the Culm. They rather seem to rest 

 on their denuded and distui'bed surfaces, and many appearances would 

 indicate that the conglomerate series overlap, or are unconformable 

 on the Lower Culm. Indeed, the conglomerate series sometimes 

 rest directly on the Devonian limestones, clearly proving that great 

 denudation of the Lower Culm, and even of the Devonian itself, had 

 occurred previous to the formation of the conglomerates. These 

 nnconformabilities were long ago distinctly noted by Godwin-Austen 



