ON THE rORPHTRTTTC ROCKS OF THE ISLA^^I> OF JERSEY. 35 



2. On the Poephtritic Eocks of the Islais^d of Jersey. By Prof. 

 A. de Lapparext, Foreign Correspondent of the Society. 

 (Eead :N^ovember 12, 1890.) 



(Communicated by the President.) 



HavijS'G been supj)"lied some years ago, through the kindness of my 

 friend the Ptev. C. Noury, of St. Heher, with a good series of speci- 

 mens of porphyritic rocks from the Island of Jersey, I was struck by 

 the exceptional appearance of some felsitic and globular varieties, in 

 which the spheroidal concretions, instead of being minute globules, 

 as in the usual pyromerides, attained a size of two feet and more in 

 diameter. According to the information which I then possessed, 

 the porphyritic rocks occupied the whole of the north-eastern part 

 of the Island, resting on Cambrian schists, and underlying, from 

 Rozel to St. Catherine's Baj', a coarse conglomerate, which had been 

 described by Ansted as New Red Sandstone. 



At that time I strongly advocated the views entertained by my 

 distinguished friend M. Michel-Levy, on the relation between 

 structure and geological age in eruptive rocks; and, judging from 

 many examples collected in various districts of France, I believed 

 that every felstone and pyromeride would be found to be of Permian 

 age. Accordingly, as this belief seemed to be warranted by the facts 

 in Jersey, I described the whole of the porphyritic rocks of the 

 Island as Permian *. Moreover, I even ventured to lecture my 

 English predecessors for having failed to recognize the true age of 

 the eruptive series, the felsites of Jersey having been described by 

 Mr. Davies as " old rhjolites." This was a mistake on my part, 

 which I now feel myself bound to confess before the Geological 

 Society of London. 



The first doubts as to the correctness of my theoretical views arose 

 in my mind immediately after the Meeting of the Geological Congress 

 in London in 1888. I had taken part in the excursion to North 

 "Wales, under the guidance of Dr. Hicks, and had there observed the 

 intercalation of true felsites in the " Precambrian slates," while 

 numerous boulders of the same felsites were to be found among the 

 constituents of the conglomerate which underlies the purple slates 

 of Llanberis. Accordingly I could no longer hesitate to believe that 

 eruptive rocks with true felsitic structure might belong to the 

 earliest geological periods. 



Some months later, my friends Mr. HiU and M. Bigot entered, 

 each for himself, upon the geological study of the northern group of 

 the Channel Islands. They both wereled to the conclusion that 

 the grits and conglomerates of these Islands, which are not to be 

 separated from the conglomerate of the north-east corner of Jersey, 

 and which also contain pieces of felsite and globular porphyry, ought 



* Bulletin Soc. Geol. France. 3"^ ser. vol. xii. pp. 284-289. 



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