332 SECTIONAL TRANSACTIONS.— C. 



Miss E. M. LiND Hendriks. — The Stratigraphy of South Cornwall. 



The so-called Ordovician sequence of South Cornwall resolves itself into two 

 series of Devonian age, the Gramscatho overlapped by the Grey-Black Beds. The 

 beds are subdivided and the distribution indicated from Porthleven to Pentewan. 



A fossiliferous quartzite of Caradoc age, previously regarded as dating the entire 

 succession, is shown to be restricted to repetitions of a crush-belt produced by lateral 

 movement operating above the plane of weakness provided by the overlap. The 

 oldest rock is therefore never ' in situ,' and always associated with the highest beds. 

 FossUiferous Upper Silurian limestones similarly occur. Quartzite fossils from 

 Gerrans Bay include Orthis altera Barrande, characteristic of Barrande's Ddo beds 

 of Central and S.W. Bohemia. New limestones from the same locality match an 

 ' Orthoceratenkalk ' in Barrande's Eej beds of Bohemia and Bavaria. 



Barrande's stages F-G (Bohemia) are represented at the base of the overlapping 

 Grey- Black series in Gerrans Bay by fossiliferous pebbles including a Lower or Middle 

 Devonian fish, and fragments of the underlying Gramscatho Beds. The Grey-Black 

 series is therefore not inverted, and cannot safely be placed lower than Upper 

 Devonian. The spUite near its top is identical with that of MuUion Island. The 

 Gramscatho Beds beneath the overlap contain throughout a new species of Dadoxylon 

 wood of a type not known below Middle Devonian, with fragments suggestive of 

 Asteroxylon Mackiei indicating that age. Tiny coal layers in the lowest division 

 compare with those in Barrande's stage H of Bohemia (Hostim, &c.). 



The South Cornish beds below the overlap are thus of French, Alpine, Central or 

 South European type, the North Europe (Rhine, &c.) facies of the North Cornish 

 Devonian being well known. 



Tectonic breccias within the crush-belt account for most of the beds mapped as 

 ' unconformable basal Devonian conglomerate ' ; the remainder comprise natural 

 breccias on the horizon of the Gramscatho tuS, and in East Meneage a small brecciated 

 complex. This is an intrusion of serpentine, continuous with a sheet in Veryan Bay, 

 injected contemporaneously with, and altered by, the movement, which is continuous 

 with the Lizard boundary-fault. At an intermediate point in Gerrans Bay the same 

 serpentine, less altered and resembUng that of the Lizard, overlies the thrust and is 

 the plutonic equivalent of a spilite not older than Upper Devonian. Both serpentine 

 and crush-belt are therefore closely post-spiUte, and earlier than the post-Carboniferous 

 stresses by which they have been repeated. The crush-belt is thus referable only to 

 a period between Upper Devonian and Carboniferous, and therefore connected with 

 the Variscian folding which, with similar strike, accompanied the injection of 

 serpentines to gabbros and granites in Central Europe. The Lizard serpentine thus 

 belongs to a Bohemian province of Variscian (Lower Carboniferous) date. It further 

 follows that the spUites of South and North Cornwall are of one age (Upper Devonian) ; 

 hence the structure of the entire county is the relic of a pinched-up fan with the 

 southernmost syncbne replaced by a Lower Carboniferous thrust. 



The beds are correlated as in the table on the opposite page. 



September 11 — 18. 



Excursion to West Cornwall. 



After the meeting closed at Bristol, some twenty-five members moved on to 

 Camborne, where a series of visits to mines, quarries and pits had been arranged by 

 Mr. E. H. Davison, of the Camborne School of Mines. The party was very warmly 

 welcomed, and all possible facilities were afforded both for geological study and also 

 of manufactitting and mining processes. 



; September 11. — The Carnmenellis Granite and the Polhigey Tin Mine. The 

 Anglo- Oriental Corporation entertained the visitors to lunch. 



In the evening a formal welcome was given at a dinner on the invitation of the 

 Cornish Institute of Mining Engineers. 



September 12. — Underground in the East Pool and Agar Mine. Messrs. Holman 

 Bros., Ltd., invited the party to lunch, and then showed the manufacture of mining 

 machinery. 



September 13. — -Castle-an-Dinas, Zennor, Gurnard's Head, Porthmeor Cove. 



September 14. — Porthleven, MegUliggar Rocks, and Prah Sands. 



