604 J". V. LEWIS OEIGIX OF PILLOWY LAVAS 



across the island. The chert is radiolarian and is int^rstratified with the 

 shale in bands from one-fourth of an inch to several inches thick, as many 

 as 30 bands of it occurring in a thickness of 3 feet. The ellipsoidal lava 

 is regarded as having the character of an extrusive : 



'The phenomena might be explained by the simple flow of a submarine lava, 



if such a lava possessed the power of insinuating itself between layers of 

 deposit and tearing them up during its onward march.'" 



Pillow lavas like those of Mullion Island were first recognized on the 

 Cornish mainland at Gorran Haven in 1901/^ and their occurrence at 

 Tregidden, Porthallow, and intermediate points was demonstrated by 

 Prior^^ in 1904. The rocks of the whole range, from Mullion Island to 

 Porthallow, are described by Flett and Hill.^' They occur in the Veryan 

 series of the Ordovician. with accompanying intrusive and tuffs. They 

 are much shattered in places along a zone of brecciation, but the original 

 igneous structure is exceptionally well preserved. Fine-grained dark 

 green rocks prevail, with small, rounded steam cavities, but highly vesic- 

 ular varieties also occur. Epidotization ha^ given rise to a bright green 

 color, as in similar rocks of Anglesey, the Lleyn, and other parts of Great 

 Britain. Among the pillows are pale chert and shreds of laminated ma- 

 terial "'that suggest films of mud incorporated by the lava in its flow over 

 the sea-bottom.*" 



Ussher^^ found pillow basalts, with '"inrolled interstitial sediment," in- 

 cluding a little chert, in the Upper Devonian districts of Saltash and 

 Devonport, Cornwall. At the Devonport workhouse quarry "detached 

 pillows with concentric lines of steam cavities are found in the neighbor- 

 ing slates." Xear Henn Point cherts terminate abruptly against the 

 igneous rocks. To the naked eye these look like radiolarian cherts, but 

 the microscopic characters do not confirm this impression. 



The great sheet of pillow lava more than 200 feet thick, near Port 

 Isaac, Cornwall, was fully described, in 1908, by Eeid and Dewey.^® As 

 these authors indicate, the peculiar concentric structure had been noticed 

 and figured by TTliitley**' as early as 1848, the lava appearing 



55 Mem. Geol. Survey of Great Britain. Summary of Progress for 1901. p. 17. 



»G. T. Prior: Geol. Mag.. 1904. pp. 447-449. 



^ J. S. Flett and J. B. Hill : The geology of the Lizard and Meneage. Mem. Geol. Sur- 

 vey of Great Britain. England and Wales. Expl. of sheet 359. 1912. pp. 31. 177-182. 

 183-185. 



38 W. A. E. TTssher : The geology of the country around Plymouth and Liskeard. Mem. 

 Geol. Survey of Great Britain. England and Wales. Expl. of sheet 348. 1907. pp. 83, 

 94-97. See also Summary of Progress for 1904. p. 25. 



^ Clement Reid and Henry Dewey : The origin of the pillow lava near Port Isaac, in 

 Cornwall. Quar. Jour. Geol. Soc. London, vol. 64. 190S. pp. 264-272. 



*o Nicholas Whitley : On the remains of ancient volcanoes on the north coast of Cam- 

 wall, etc. Thirtieth Ann. Kept, Roy. Inst Cornwall, App, vi, 1848-1849, p, 62. 



