GEOLOGY 



the district south of Oldchard and Ideford, and in the faulted outliers 

 between Newton Abbot and Ipplepen, beds of coarse and fine grey- 

 wacke, associated with dark argillaceous shales, contain conglomerates in 

 which fragments from the Lower Culm cherts and hard mudstones are 

 distinguishable in association with materials derived from granitoid and 

 volcanic rocks. These beds rest on the upper horizon of the Lower 

 Culm, but sometimes on dark shales which may be locally their own 

 lower horizon. 



At EfFord and Lower Compton (near Plymouth) even beds of grey- 

 wacke with shaly partings and occasional small dark shale fragments rest 

 on dark shales, upon Upper Devonian slates associated with volcanic 

 rocks. The greywackes are generally coarse and are largely made up 

 of materials derived from gneissose, and many varieties of volcanic rocks. 



At Tavistock dark Middle Culm greywacke displays a similar de- 

 rivation. At Beer Alston station a patch of grey chert underlies an 

 outlier of plant-bearing greywacke and shales. This evidence is cor- 

 roborated by the greywackes of Wearde (south of Saltash) which rest 

 on Upper Devonian slates and volcanic rocks. It proves conclusively 

 (unless we regard the unconformable greywacke as contemporaneous 

 volcanic grits of Upper Devonian or Lower Culm age) that a consider- 

 able unconformity exists in part of south Devon and southern Cornwall 

 between the Middle and Lower Culm, accounting for the complete 

 destruction of the latter and the incorporation of its materials, together 

 with volcanic rocks in abundance, in the newer sediments. Whilst the 

 Middle and Upper Culm rocks contain plants, said to connect them with 

 the calciferous sandstones of the north, the fine grained sediments and 

 Radiolarian cherts of the Lower Culm denote deep water. It is there- 

 fore less remarkable that there should be a marked unconformity between 

 them, than that their succession should be so uniform and regular over 

 the greater part of the Culm area. Whether this is due to a gradual 

 elevation not shared by those parts of the sea bed where the crust had 

 been weakened by igneous intrusions and late Devonian and early Carbon- 

 iferous volcanoes (such as Brent Tor), or to irregularities in the sea bed, 

 due to the outpouring of igneous rocks from such volcanoes and to dis- 

 turbance thereby of existing sediments, opens a wide field for conjecture. 



Upper Culm. — The Upper Culm Measures, owing to their massive 

 even bedded character, dark shale partings and pale grey colour, exhibit, 

 in the coast section near Clovelly and in the exposures by the river 

 Torridge, more beautifully than any other series of rocks in Devon the 

 numerous curves that prevail everywhere throughout the Palseozoic area. 

 This ability to undergo contortion without snapping makes their chief 

 value as the index of the axis of the great Palaeozoic trough and amply 

 justifies their separation from the Middle Culm. 



Although the term greywacke might be applied to most of the Culm 

 and many of the Devonian grits its use is here restricted to rocks of a 

 markedly felspathic character and betraying distinct derivation from vol- 

 canic as well as sedimentary materials. 



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