THE GEOLOGICAL AGE OF CENTRAL AND WEST CORNWALL. 31 



I am not aware that any of the other remains found in the 

 limestone have been named at all. Like other ancient limestones 

 some of these which, to the unassisted eye, shew no signs of 

 organisms, when viewed under the microscope in thin sections 

 are found to be full of them. 



These limestones are found at Nelly's (or Betsy's) Cove, one 

 mile north of Porthalla,'^" at Gerrans Bay,f at Polgrain in the 

 parish of Caerhayes,| and at Gorran Haven. 



The schistose rocks of this series have yielded a few corals, 

 the most common among them being Petraia Celtica. The dark 

 slates of the Black Head have yielded Graptolites, according to 

 Mr. Peach, § and very recently my son has found what appears 

 to be a Favosites in a soft greyish schist at Lower Newham, near 

 Truro, which appears to be of Lower Silurian Age.|| 



* Trans. Roy. Geol. Soc. Corn. IX, pp. 11, 12 j President's Address, 1870, 

 and X, p. 52. 



t ibid. X, p. 5, 1878. 



X The thickest mass which I have seen crosses the road at Polgrain, near 

 the church of St. Michael Caerhayes. There are here two beds — the largest 

 being about 20 feet thick. In a cavity I found crystals of Aragonite — a rare 

 mineral in Cornwall. 



§ Trans. Boy. Geol. Soc. Corn. Ill, p. 121. 



II Mr. P. M. Duncan was kind enough to examine some of the specimens of 

 this coral which I sent to him — and to write me upon them as follows : — 



" The schist was very difficult to manage, but I got some sections at last the 



markings on the surface of the planes of the schist greatly resemble infiltrated 

 coraUites of Favosites or some other tabulate Hydrocorallae — moreover, where 

 the markings are oblique they fit in very well as far as appearance goes, with the 

 walls and tabulse of the genus. No perforations are indicated. On the whole, 

 I should say it was a metamorphosed coral." Examined m situ this supposition 

 becomes greatly strengthened. The markings are seen. to pass through many 

 layers of the schist, and sometimes occur in distinctly formed semi-circular 

 masses. 



The schist is extremely fragile, but organisms occur very plentifully in a 

 thin bed of considerable extent. If this be an ancient coral reef, of which there 

 can be but little doubt, the analysis which I have made indicates a very remark- 

 able amount of metamorphism, only 0"8 per cent, of lime being now present in 

 the stone. At the same time fJiis is a considerably higher proportion than is 

 usually present in the rocks of the neighbourhood. (See Journ. Roy. Inst. Corn. 

 XXX, p. 408). 



The accompanying list includes all the Cornish Lower Silurian fossils as far 

 as they are known to me — as well as several which are still doubtful. 



The references are to the Trans. Boy. Geol. Soc. Corn., except where 

 otliersN'ise stated. 



