276 C. Calkncay — L. Helderherg Groiqo of N. York. 



ground for extending the Ludlow upwards so as to include the 

 Hamilton group, as some American geologists are disposed to do. 



The formation underlying the Lower Helderberg being the 

 equivalent of the Wenlock, and the formations overlying being 

 clearly correlated with our Devonian, it is fair to conclude that the 

 Lower Helderberg group represents our Ludlow. 



A comparison of the Ludlow and Lower Helderberg faunas will 

 not supply us with additional evidence of a very decisive character. 

 Putting on one side the exceptional case of the Conchifera, the 

 Ludlow fauna has no close resemblance to any American formation. 

 Of individual species, we may compare Phacops (Dalmannia) longi- 

 caudatus with the much, larger Dalmannia nasuta, Bhynchonella 

 Wilsoni with B. ventricosa, and Strophomena euglypJia with S. punc- 

 tulifera. In the Aymestry Limestone near Broseley, I have also 

 detected OrtMs piano -convexa of the Lower Helderberg. But of the 

 numerous Cephalopoda of our Lower Ludlow, we have few, or no, 

 representatives in the American area. On the whole, the Lower 

 Helderberg fauna resembles that of the Niagara series of New York 

 more closely than it resembles that of the British Ludlow. The 

 explanation of this is not far to seek. At the close of the Niagara 

 period, the sea shallowed, and salt lakes were formed. But this 

 elevation was followed by a marine epoch, during which the Niagara 

 fauna, driven into adjacent seas by this local upheaval, would return 

 to their old habitat, modified in form to such a degree as we might 

 expect from the interval of time represented by the Salina period. 

 The Lower Helderberg corals and molluscs flourished in the same 

 area as, and on a similar sea-bottom to,. the Niagara fauna. But in 

 the British area the conditions were different. During the Ludlow 

 period, the sea was gradually shallowing, so that towards its close 

 the proximity of land is indicated by arenaceous strata and the 

 presence of land-plants, and of fishes, as Pteraspis, which probably 

 inhabited fresh- water. A change from a calcareous to a sandy sea- 

 bottom, and from marine to land and fresh-water conditions, would 

 produce a change in the fauna much more rapidly than a mere lapse 

 of time. In the New York area, terrestrial conditions do not set in 

 till the Hamilton period, and, with the coming in of a sediment 

 similar to the Upper Ludlow rock, we have the advent of the 

 Ludlow Lamellibranchs. But where the Hamilton rocks lose their 

 arenaceous character, and pass into calcareous strata, as in Western 

 New York, the Lamellibranchs grow scarce, and the prevailing 

 forms of life are Brachiopoda and Corals of Devonian types. 



In correlating rock systems, the work is comparatively easy when 

 the physical conditions under which their faunas flourished were 

 similar. We can compare arenaceous Coal-measures in Europe with 

 arenaceous Coal-measures in America; or a calcareous Wenlock with 

 a calcareous Niagara, and our conclusions are clear and satisfactory. 

 But changes in sediment, and other physical causes, introduce great 

 complexity into the evidence. I do not, however, despair of obtaining 

 true results when, as in the above case, we know the formations 

 above and below the groups which we wish to correlate. Our base- 



