228 Revieics — Geology of Tavidock and Lauuceston. 



y)acking, and overtlirusting, the true sequence of the several sub- 

 divisions in the Upper Devonian is not manifest, nor are the relations 

 clear between Devonian and Lower Culm Measures, nor between the 

 Lower and Upper Culm Measures. The general sequence, however, 

 appears to coincide very fairly with that given by John Phillips from 

 the Upper Devonian Petherwin Group with Goniatites and Clymenia, 

 to Carbonaceous or Culm Measure shales, shales with limestone, and 

 Coddon Hill chert series; and he observed appearances of some 

 unconformity between the base of the Culm Measures and the 

 Petherwin Group. The extension of the chert beds is well shown on 

 the new map by a bright yellow tint, and they are regarded as 

 probably belonging to the Ui:»per Limestone Shale, a position equivalent 

 to that of the Pendleside Series, assigned to the Coddon Hill Beds by 

 Dr. Wheelton Hind.' With regard to the origin of the chert, the 

 authors of the present memoir remark that "the chert seems to be an 

 altered shale of very fine grain, containing a considerable number 

 of radiolaria, but geiierallj'^ without the calcareous foraminifera and 

 other calcareous organisms which must have accompanied them. 

 The preservation of the fossils was apparently due to siliciiication 

 of the rock at an early date, before the radiolaria had disappeared, 

 but after tlie calcareous organisms had been removed ". In this view 

 the authors differ from Dr. G. J. Hinde and Mr. Howard Fox, who 

 regarded the cherts as " essentially of organic origin ". 



There is much of interest in the accounts of the volcanic rocks with 

 pillow-lavas in the Upper Devonian of Tavistock and in the Lower 

 Culm Measures of Brent Tor and elsewhere. That some of the 

 igneous areas are complicated enough may be judged from the 

 description of a large road-stone quarry at Newbridge, south-west 

 of Callington. There " in the quarry itself the rocks are seen to 

 be greatly disturbed. At its entrance an outcrop of slate has been 

 converted to spilosite through contact with an intrusive basic igneous 

 rock which appears through the floor of the quarry on the western 

 side. Chert is found in the lava on the eastern side near a small 

 fault. Above the spilosite, on the western side, lava and chert are 

 in juxtaposition, but the junction between them and the slate is 

 a disturbed one. Further in the quarry this lava is found to be 

 a prolongation of the main mass. A little spilosite is found above 

 the lava, but the rest of the quarry inwards consists almost entirely of 

 fresh vesicular lava ". 



The association of contemporaneous volcanic rocks with the Culm 

 Measure chert is interesting, and suggests to the authors some possible 

 connexion between the submarine eruptions and the silicification before 

 mentioned. The greenstones, the polyphant stone (a serpentinized 

 picrite),- the granites and elvan-dykes, and the metamorphic aureole 

 are duly described. 



There is no great amount of valley drift, but it comprises gravel, 

 alluvium, and peat, and there is also some hill-peat. Stream-tin and 



1 Geol. Mag., 1904, p. 400. 



^ The paper by Mr. J. H. Collins on Cornish Serpentinous Rocks, Geol. Mag., 

 18S6, p. 366, has apparently been overlooked; see also Collins, Journ. R. Inst. 

 Cornwall, vol. xxiii. 



