88 



PROF. T. M C KENNY HUGHES ON THE 



Mag. dec. 2, vol. iii. 1876, p. 526 ; Strahan, Mem. Geol. Survey, 

 " Geology of Chester " ; Mellard Eeade, Quart. Journ. Geol. Soc. 

 vol. xxxix. 1883, p. 92 ; Wood, S. V., Geol. Mag. dec. 2, vol. iii. 

 1876, p. 396, footnote). 



But along the east coast, both north and south of the Wash, 

 marine conditions prevailed long before the ice had receded into 

 the high mountains, and in the shells of the older drift we find the 

 record of this. 



I have not entered into the discussion of the exact subdivisions or 

 correlation of these beds. I have referred to some authorities for 

 just the point above stated of an older Glacial and a newer Post- 

 glacial drift. For local purposes a more minute subdivision is 

 possible and useful, but at present I think we must, for wider cor- 

 relation, adhere to a simpler system, and speak of one great gla- 

 ciation succeeded by one great submergence in the west ; and in 

 the east, an older marine, probably synchronous with the first or 

 land-ice, and a newer marine, the chronological equivalent of the 

 second or age of submergence in the west. For some suggestive 

 remarks on this subject see Jamieson, " On the Cause of the Depres- 

 sion and Re-elevation of the Land during the Glacial Period," Geol. 

 Mag. dec. 2, vol. ix. 1882. (See also Dakyns, " Glacial Deposits 

 north of Bridlington," Yorksh. Geol. Polytech. Soc. vol. vii.) 



In the subjoined table (p. 93) I have given a list of the shells in the 

 St. Asaph Drift and from beds in adjoining districts, which it appears to 

 me belong to about the same age. I have added in the last column 

 for comparison a list of the shells from what is probably a marine 

 deposit of the age of the Arenig land-ice. 



By reference to the authors quoted it will be seen that in many 

 cases there are subdivisions of importance in the beds which I have 

 included under one head, and that some of the forms which I have 

 recorded may have been derived from older deposits. It is clear 

 also, from the great difference of elevation, lithological character, 

 and proximity to the mountains, that a somewhat different facies may 

 be expected from deposits which have been laid down at different 

 times in one long age of changing geographical conditions and 

 climate. But nevertheless this point seems to be clearly estab- 

 lished, that the shells enumerated in columns I. to VII. all belong 

 to a somewhat northern temperate group (i, e. are Post-Glacial), 

 that under the deposits in which they occur on the west are the 

 land-ice drifts of the Welsh and Lake-country mountains, and 

 that below their equivalents in the east are Old Boulder-clays con- 

 taining, either scattered through the mass or in included masses of 

 contemporaneous sand and clay, a severely Arctic group of shells. 

 These shells are recorded in column VIII. 



I have omitted all notice of the Echinoidea or Foraminifera, as 

 not being of sufficiently common occurrence to be useful for our 

 present purpose of correlation. 



The references to the formation, locality, and age, and the sources 

 of information are as follows : — 



In column I. are indicated the shells found in the marine sands 



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