558 T. Mellard Meade — The Dimetian of St. Davids. 



strong resemblance to one another, and, considering the long lapse of 

 time Fusus antiquus has lived, the many geographical positions it has 

 occupied, the different conditions, climatal and otherwise, in which it 

 now exists, is there not reason to conclude that it is the original 

 parent stock from which most of the species now inhabiting the 

 Northern Hemisphere have branched forth? 



The New England shell called Fiisus tornatus by Gould (Inv. 

 Mass. 2nd edit. p. 37) closely resembles some examples collected at 

 Bramerton, near Norwich; it is recognized by him as resembling the 

 Crag fossil ; its relation to Fusus despectus, L. (which again it is 

 almost impossible to divide from the carinated form of F. antiquus) 

 is acknowledged by later authorities, such as Verrill, Sars, and Friele. 



.In the Red Crag, however, the carinations are seldom, if ever, 

 shown so sharply as in the last-named shell, possibly from abrasion 

 as in the case of most Crag Casteropods ; in their present aspect with 

 their strong coarse carina, they nearly resemble the older shells of 

 F. liratus, Martyn, a native of Behring Sea. F. fornicatus of Linne, 

 which has the carina coarse and undulating, has not been met with 

 in the Crag, although sought for diligently by the late Mr. Wood. 



On the Pacific Coast of North America Fusi are scarce in number, 

 F. tabulatus of Vancouver not having much resemblance to our shell ; 

 but on the opposite coasts of North-Eastern Asia varieties of the 

 common form occur in the seas of North Japan. The Aleutian pro- 

 montory and group of islands probably deflecting many of the Arctic 

 Molluscs inhabiting deep water to the Asiatic side, where they are 

 carried south by the main current of water which sets that way. 



The fragmentary character of the geological record is very apparent 

 in the Pliocenes of Northern Europe, but their total absence in North 

 America, and our limited knowledge relating to any of these deposits 

 (small in extent as they are) outside the British Islands, imposes great 

 difficulty and consequent imperfection in such life studies as have 

 been attempted above. 



VII. — The Dimetian of St. Davids. 



By T. Mellakd Eeadb, F.G.S. 



HAVING been lately at St. Davids, I made some observations in 

 relation to the Porth-lisky granite or Dimetian of Dr. Hicks, 

 which may be worth recording in the pages of the Geologioal 

 Magazine. 



Perhaps it may be well for me to state that previous to my 

 examination I had no bias whatever in favour of any of the explana- 

 tions of the geology of St. Davids which have been offered by 

 Hicks, Geikie, or Blake. 



With Dr. Hicks's map in hand ^ I examined many of the contacts 

 of the granite with the surrounding rocks, but could see no evidences 

 of any faulting sufficient to justify its title to be recognized as a 

 distinct Archaean formation. 



The evidences I did collect, as will be seen in the sequel, tend to 



^ Pre-Cambrian Eocks of Pembrokeshire, QJ.G.S. vol. xl, p. 560. 



