ON THE STOCKBALE SHALES. 719 



do find Graptolites rarely preserved in the mudstones, and though 

 we have hitherto found no Trilobites in the Graptolite-shales, occa- 

 sional Brachiopods and corals have turned up. In Dalecarlia one of 

 us has seen Trilobites preserved in some of the Graptolite-bearing 

 representatives of the Stockdale Shales. Such a lingering- on under 

 unfavourable conditions would be admirably qualified to bring about 

 that variation in the creatures which would account for the marked 

 contrast between the fossil content/S of beds separated only by a few 

 feet of intervening rock. 



On the other hand, the occurrence of a zone in Scania with an 

 intermixture of the forms of the zones of Petcdograptus cometa and 

 MonograiAus sjphmjerus tends to indicate that there may have been 

 also migration from one region to another during a time unfavourable 

 to the existence of a group of organisms in the former, and that in 

 this way an intermixture of two faunas elsewhere separated would 

 result, but our present experience tends to show that this is some- 

 what rare. 



With regard to the age of the Stockdale Shales, we shall say little, 

 as we consider the question fully settled. They are conformable to 

 the Ordovician beds below and to the Wenlock beds above ; and this 

 indicates that they represent the two Llandovery subdivisions and 

 the Tarannon shales, in other words that they belong to the series 

 for which Professor Lapworth has suggested the name Yalentian. 



Our comparison of the beds with those of other areas entirely 

 supports this view. The Birkhill Shales have been referred by 

 Professor Lapworth to the Lower Llandovery, in his paper on ^' The 

 Moffat Series." But the same author has also shown that the Gala 

 Beds, the equivalents of our Browgill Beds, represent the Tarannon 

 Shales ; in which case, the Birkhill Shales and the corresponding 

 Skelgill Beds must include representatives of both Lower and Upper 

 Llandovery ; and this is the view taken by Professor Lapworth, in 

 his subsequently published paper on " The Geological Distribution 

 of the Rhabdophora," where he describes the Birkhill Shales and 

 Gala Group under the title "Valentian or Llandovery-Tarannon 

 formation." 



The fossils of the Stockdale Shales support this to the fullest ex- 

 tent. Many of the Graptolites and of the higher organisms are 

 exclusively limited to representatives of the Yalentian formation in 

 other areas, and there are very few which transgress the limits of 

 this group. 



The most important result of our researches is the additional 

 evidence which we have furnished of the value of Graptolitic zones 

 as a means of comparison of Lower Palaeozoic rocks of distant areas. 

 We have long looked with admiration on the remarkable results of 

 Professor Lapworth's detailed researches upon these rocks, and have 

 for many years been convinced of the importance of his results. 

 We have watched with pleasure the adoption of his views and of 

 his methods of working by the enthusiastic geologists of Scandinavia. 

 We must confess, with disappointment, that we have frequently heard 

 British geologists express themselves in words of hesitation con- 



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