I902. HiNCH. — Glacial Geology of Co. Dubli7i, 233 



when touched. At Larch Hill, on the contrar}^ all the shells 

 from the upper pit could be handled quite as freely as recent 

 ones, many retained some of their original colouring, and a 

 number of the Gastropoda were perfect. This retention by 

 the shells of texture and colour, appears to me to be the result 

 of the protection afforded to the granite boulder bed by the 

 overlying Boulder-clay : the Boulder-clay band is practically 

 impervious to percolating water, and this prevents the surface 

 drainage from reaching the shells and destroying them in the 

 usual manner. This is well seen after heavy rain, when the 

 water from the overlying sand and gravel either accumulates 

 in the inequalities of the Boulder-clay band or runs off it in 

 streamlets : another proof of this protection by the Boulder- 

 clay is the fact that in the lower pit, where the Boulder- clay 

 thins out, the shells are in the chalky condition of other 

 localities, though much more perfect. 



The shells, as a whole, indicate exposure to very rough 

 treatment. The extent to which this has taken place can be 

 understood by the fact that out of over 2,000 fragments col- 

 lected, only 13 perfect Gastropods, and in the Lamellibranchs 

 18 perfect valves, were obtained ; and a reference to the list 

 will show that it is only the small, compact shells which have 

 escaped destruction. Cyprina islandica, which might be 

 expected to resist great pressure, is represented by over 100 

 fragments under an inch in length, the powerful hinge being 

 the only part of the shell strong enough to escape destruc- 

 tion. 



Excluding doubtful species, in the Larch Hill list we have 

 a group of 35 mollusca — 19 Lamellibranchs and 16 Gastro- 

 pods ; of these, three species {Astarte borealis, Leda pernula^ 

 and Trophon clathrahis) are arctic shells not now found in 

 the British seas. Five species {Astarte compressa, Pleurotojna 

 turriculaj Glychneris \Panoped\ nofvegica^ Cyprina islafidica, 

 and Trophon Bartiffius) have at the present time outside the 

 British seas a completely northern range ; the remaining 27 

 species have a distribution both north and south of Britain. 

 It will be seen that the fauna has a distinctly northern facies, 

 the arctic and northern shells constituting nearly one-fourth 

 of the whole list, and pointing to a much colder climate than 

 at present obtains in the Irish sea area. 



