102 PKOCEEDINGS OF TFE GEOLOGICAL SOCIETY. 



any other observer, have shown the abundance of volcanic material 

 among the rocks of Devonshire, and the resemblance which in this 

 respect they offer to the Devonian system of. North Germany.^ 

 Unfortunately the geological structure of the Palaeozoic rocks of 

 the South-west of England has been complicated to an amazing 

 extent by plication and fracture, with concomitant cleavage and 

 metamorphism. Hence it is a task of extreme difficulty to trace 

 out with any certainty definite stratigraphical horizons, and to deter- 

 mine the range of contemporaneous volcanic action. Mr. Ussher 

 has recentl}^ shown us with w^hat success this task may be accom- 

 plished when it is pursued on a basis of minute mapping combined 

 with a sedulous collection and determination of fossils.^ Eut years 

 must necessarily elapse before such detailed work is carried over the 

 whole Devonian region, and probably not till then will the story of 

 the volcanic history of the rocks be adequately made out. 



In the meantime it has been established that while there is a 

 singular absence of igneous rooks in North Devon, a strip of 

 country extending from Torquay to Plymouth contains abundant 

 records of contemporaneous volcanic action. It has not yet been 

 possible to map out the respective limits of the bedded lavas and 

 the tuffs, to distinguish the true sills, and to fix on the position of 

 the chief vents of eruption. So intense has been the compression 

 and shearing of the rocks that solid sheets of diabase have been 

 crushed into fissile schists, which can hardly be distinguished from 

 tuffs. Moreover, owing perhaps in large measure to the mantle of 

 red Permian (or Triassic) strata, which has been stripped off by 

 denudation from large tracts of this region which it once overspread, 

 the Devonian rocks have been deeply ' raddled.' But probably one 

 of the main sources of difficulty in studying the petrography of the 

 area is to be found in the results of atmospheric weathering. Devon- 

 shire lies in that southern non-glaciated strip of England where the 

 rocks have been undergoing continuous decay for a protracted period, 

 and where no ploughshare of ice has swept off the deep weathered 

 crust so as to leave hard surfaces of rock, fresh and bare, under a 

 protecting sheet of boulder-clay. It is seldom that a really fiesh 



^ See in particular bis last paper ' On the Ashprington Yolcanic Series of 

 South Devon,' Quart. Journ. Geol. Soc. toI. xlv. (1889) p. 369. 



2 This geologist has spent many laborious years in the investigation of the 

 geology of Devonshire, and has published numerous papers on the subject. 

 The valuable memcir by him specially referred to above will be found in 

 Quart. Journ. Geol. Soc. vol. xlvi. (1890) p. 487. 



