34 



MULLION ISLAND. 



By HOWARD FOX, Esq., F.G.S., President of the R.G.S. of Cornwall. 



In the fourth volume of the Transactions of the Eoyal 

 Geological Society of Cornwall published 60 years ago, Dr. H. S. 

 Boase gives the result of two years unceasing labour in exploring 

 the geology of our county, and at page 344 we read the words 

 ' ' I did not go on Mullion Island, but, from the nearest headland, 

 it appears to be composed partly of serpentine and partly of 

 hornblende rocks." In the map, which accompanies his paper, 

 he wisely leaves the island uncoloured. I can find no other 

 reference to Mullion Island in any of the eleven volumes of this 

 Society's Transactions. 



The map of Cornwall published by the Geological Survey 

 does, however, colour this island, and colours it as if it were 

 of serpentine. There is, nevertheless, not a fragment of serpen- 

 tine or hornblende schist apparent in the island. 



In August of last year, accompanied by my friend Mr. E. N. 

 Rogers, I was able to carry out a long planned trip to the island. 

 Finding rocks there with which I was unfamiliar, I sent some 

 specimens to be sliced for the microscope, and forwarded them 

 to Mr. J. J. H. Teall, F.R.S., who kindly examined them and 

 reported that two were from sedimentary rocks, one of which 

 appeared to be a radiolarian chert and the other a limestone. 

 Further specimens from the island were sliced, and the fact was 

 verified that Mullion Island contained Eadiolarian Cherts ! 



Mr. Teall came to Cornwall the following November, accom- 

 panied me to Mullion and found the main mass of the island to 

 be composed of a fine-grained "greenstone" of a peculiar 

 globular or ellipsoidal structure associated in certain parts of 

 the island with bands, sheets, and lenticles of chert, shale, and 

 limestone, as indicated in the map annexed. He moreover 

 discovered that the reticulated structure of the individual radio- 

 laria could be clearly seen by placing the weathered surface of 

 some specimens under the microscope, whilst even to the naked 

 eye they were apparent as minute specks thickly dotting the 

 light coloured edges of the dark chert bands. An ordinary 

 pocket lens shows each of these specks to have a circular forn^. 



