712 DE. HELGI PJETURSS0N ON [Nov. I906, 



31. The Crag of Iceland — an Intercalation in the Basalt- 

 Eormation. By Dr. Heloi P.tetuesson. (Communicated by 

 Prof. W. W. Watts, M.A., M.Sc, F.R.S., Sec.G.S. Head 

 June 13th, 1906.) 



The existence of fossiliferous deposits on the western coast of Tjornes 

 (Northern Iceland) has been known to science for nearly 160 years. 

 They were first mentioned by the famous Eggert Olafsson in his 

 book entitled ' Enarrationes historicss de Islandiae datura & Consti- 

 tutione,' etc., published at Copenhagen in 1749. 



In 1871 the Danish conchologist 0. A. L. Morch published a 

 paper * On the Mollusca of the Crag-formation of Iceland,' 1 wherein 

 he enumerated 61 species of molluscs, and arrived at the conclusion 

 that in the Crag-period the temperature of the northern coast of 

 Iceland must have been much milder than now, ' at least as at 

 present on the west coast by Keikiavik ' [Reykjavik], op. cit. p. 394. 



Some years later, Mr. J. Starkie Gardner collected 33 species of 

 Pliocene molluscs in Iceland, which were studied by Gwyn Jeffreys 

 and Searles Y. Wood. In the opinion of Wood, the deposits in 

 question cannot be younger than Middle lied Crag, while Jeffreys 

 thinks that they are somewhat younger. Gardner himself is, 

 however (Quart. Journ. Geol. Soc. vol. xli, 1885, p. 97), 



' inclined to assign a greater age to the deposit from its general appearance on 

 the spot, than Dr. Gwyn Jeffreys may do, or even than Mr. Searles Wood.' 



The most important paper in existence on the fauna of the Ice- 

 landic Crag dates from 1884, but only exists in a manuscript, which 

 has been deposited in the archives of the Geological Museum of 

 Copenhagen. It is written by C. M. Poulsen, and is in part founded 

 on the investigations of Morch. Poulsen, in this MS. — which 

 was kindly lent to me for perusal by Prof. Ussing, — enumerates 

 117 species of molluscs from the Crag of Tjornes ; out of this number 

 20 species are new, while of the remaining 97 species 18 per cent, 

 are extinct. Poulsen's somewhat startling conclusion is, that the 

 Crag of Iceland is younger than even the youngest division of the 

 English Crag. 



It seems not improbable that the 117 species of molluscs recorded 

 from Tjornes in 1884 constitute a mixture of different faunas, and 

 possibly the Icelandic Pliocene deposits, which, indeed, are very 

 much thicker than has been generally assumed, fall into several 

 divisions. 



Thus Gardner and Poulsen may both of them be in part right. 

 Looking at the percentage of extinct species, it seems not improbable 

 that a portion of the Pliocene fauna of Tjornes may go farther back 



1 Geol. Mag. vol. viii, p. 391. 



