292 Professor E. J. Garivood — 



on " The Origin of Freshwater Fauna " in The Age of the Earth, 

 remarks " Paludina traces its descent from the ancient stem of the 

 Trochids and Turbonids which, as already mentioned, are found in 

 Cambrian sediments. Tt may possibly have originated in Devonian 

 lakes, but its absence from Carboniferous and Permian rocks would 

 suggest a later date." The precise age of the Viviparus Shale at 

 Horton is not easy to determine. It was evidently deposited in a 

 depression on the land surface which was being eroded during 

 Devonian times, and before that surface was submerged under the 

 Carboniferous sea. From this poijit of view it might be claimed as 

 being of Devonian age. The question is in reality a far-reaching one. 

 During the submergence which ushers in a new geological period, 

 certain areas may remain for a very long time above the level of the 

 sea in which the marine deposits belonging to the new period are 

 being laid down in. the neighbourhood. The conditions on the 

 surviving land surfaces will, however, represent a continuation of 

 those which prevailed during the preceding continental period, and 

 the question arises — at what mom.ent does the land surface cease to 

 belong to the earlier period and form part of the new period ? In 

 the case before us a considerable area of country in the north of 

 England remained above the sea while lower tracts of land were 

 being submerged in the south-west province and elsewhere under 

 the Tournaisian sea. In the Horton district, the old Devonian land 

 surface does not appear to have been inundated until the time when 

 the Cyrtina carhonaria and N. mimis beds were being laid down. It 

 is therefore impossible to determine the exact period when the 

 Viviparus Shale was deposited on the continental surface, and there 

 is no means of deciding whether this freshwater deposit should be 

 classed as Devonian or Carboniferous. 



The Lamellihranch Limestone. — The Lamellibranch Limestone, 

 overlying the Viviparus Shale, is also a thin bed, an inch or two in 

 thickness, but differs essentially from the bed below as regards its 

 petrographical character and its fossil contents. The two beds 

 appear to have been formed under very different physical conditions. 

 The Viviparus Shale, as stated above, represents apparently a 

 freshwater deposit form^ed on the surface of the old Devonian 

 continent, while the overlying Lamellibranch Limestone was 

 evidently a shallow-water deposit laid down in a strait or estuaiy 

 connected with the advancing Carbon if eroiis sea. 



The rock consists of a black crystalline limestone, which in thin 

 sections is seen to be made up of shell fragments, together with a 

 few tests of Endothyra and occasional remains of bryozoa. The 

 matrix is composed of fine-grained granular calcite in which are 

 embedded a few small angular grains of quartz. The character of the 

 deposit is therefore very different from that of the underlying 

 Viviparus Shale, which is devoid of calcite and organic remains. 

 The fossils are well preserved and consist chiefly of large well-grown 

 lamellibranchs which include the following forms : — 



