Alex Somervail — Origin of Dartmoor Granite. 609 



V. — On the Age and Origin of the Granite of Dartmoor, 



AND ITS EeLATIONS TO THE ADJOINING StRATA. 

 By Alex Somertail.' 

 rpHE object of this paper is an attempt to furnisli proof of what 

 X has been a growing conviction in the mind of the writer, that 

 the true age of the Dartmoor granite, and probably its associated 

 line of bosses running south-westwards into Cornwall, might be 

 referable to an interval or period of geological time between the 

 Lower and the Upper Culm, or Carboniferous system. 



Up to the present time, as geologists are doubtless well aware, 

 these granite bosses have been considered Post-Carboniferous and 

 Pre-Triassic as to age, and the evidences for this so ably advanced 

 by De la Beche and others have been almost universally accepted. 

 Up to Lower Culm, or Carboniferous times, the nature of the proofs 

 have been of so decisive and convincing a kind as to place the 

 question beyond doubt to the minds of most observers. There are, 

 however, grave doubts and difficulties with regard to the Permian 

 age of the granite. There are no clear proofs that it even belongs 

 to an early portion of that formation ; or that it can in any way be 

 connected with the highly basic lavas of the adjoining Permian 

 strata ; indeed, the evidences for the very reverse of this is the case. 



In support of this contention the writer would draw attention to 

 a point which has never before been urged, which appears to him to 

 very materiallj^ affect the question as to the true age, not only of the 

 granite of Dartmoor, but also of the other bosses of the same rock 

 already referred to. 



In the history of the Culm rocks of South Devon there is what he 

 considers to be a striking gap or break in the sequence between the 

 Lower and the Upper Culm, which without doubt would indicate 

 a prolonged interval of time between these two members of the 

 system, as will immediately appear. 



In making use of the term Upper Culm he would refer more 

 especially to certain conglomerates, grits, and sandstones, which seem 

 only to have a very local development in South Devon. They occur 

 as mere isolated or semi-detached deposits confined to a very limited 

 area in the neighbourhood around Newton Abbot as a centre. The 

 most conspicuous of these deposits are certain conglomerates — 

 coarse, medium, and fine — associated with which are grits and 

 sandstones. These conglomerates are not met with in any of the 

 other extensive general sections where the Lower Culm beds occur, 

 as in the direction of Exeter and the Teign Valley .Neither are they 

 known to occur in the North of Devon. In point of fact these 

 conglomerates show every indication of belonging to a much higher 

 series of beds than any that are elsewhere developed in other 

 portions of the county. 



There is a striking lithological contrast presented between this 

 conglomerate series and all the other older members of the Culm 

 in their relations to the granite. The older members are everywhere 

 affected by great earth-movements, bent and plicated sometimes to an 



^ Eead before Section C (Geology), Britisli Association, September, 1898. 



