224 PROCEED IXGS OF THE GEOLOGICAL SOCIETT. [Feb. 21, 



close of the Upper Llandeilo period in the British area. Eight of 

 these hold their ground in their own area and, as we shall see, pass 

 on into the younger deposits of the Gala group of the south of 

 Scotland (Lapworth). The whole sixteen reappear also in the 

 Conisto]! mudstones of the north of England (Caradoc). It is 

 quite clear, however, that though eight species of a total of thirty- 

 six, or twenty-two per cent., continued to exist in their original 

 area in the south of Scotland, a great migration must have accom- 

 panied an almost equally great extinction of species, marking the 

 close of the Upper Llandeilo period in the Scotch area. Seeing that 

 so large a number of forms, as above mentioned, succeed in retaining 

 their hold on their own area, it is difficult to assign any adequate 

 cause for this migration ; but that such an exodus actually took place 

 seems certain from the two following considerations : — In the first 

 place, all the species of the total thirty-six inhabiting the Upper 

 Llandeilo area of the south of Scotland, which did not actually die 

 out, are found, to the number of sixteen, in the mudstones of the 

 Coniston series of the north of England. Now these beds overlie 

 the Bala or Coniston Limestone ; and the Upper Llandeilo rocks of 

 Scotland underlie the same formation (Girvan Limestone). It seems 

 certain, therefore, that the species of the Coniston mudstones were, 

 to the above extent, derived from the Scotch area b}^ migration ; and 

 traces of such a migration are to be found in the Girvan Limestone 

 itself. In the second place, six species of the Scotch area are found 

 in strata of Caradoc age in Ireland. Five of these, and probably the 

 sixth, are also found in rocks of about the same age in North America ; 

 so that they reached Ireland en route for the American ai'ca, and the 

 former country was merely the first halting-place in their migration. 

 C. Coniston Akea op the Noeth oe England. — The above con- 

 siderations lead me to the consideration of the second great Graptolitic 

 period of the north of England, in which the mudstones of the 

 Coniston series were deposited. "Whatever may have been the sequence 

 of phenomena at the close of the Skiddaw period, it is certain that 

 the Skiddaw slates are overlain by a great mass of rocks in which no 

 Graptolites are found. That this is really due to the absence of 

 Graptolites from this area, and not to the general igneous activitj^ 

 shown by the nature of the rocks in question (Green Slates and 

 Porphyries), is proved by the occurrence in the series of a dense mass 

 of finely levigated dark-coloured shales, crowded with fossils, and 

 perfectly suitable for the preservation of Graptolites, had such existed. 

 The rocks to which I allude are the " Dufton Shales," which underlie 

 the Coniston Limestone, and which contain no Graptolites. The 

 Coniston Limestone itself, which corresponds with the Bala Limestone 

 of Wales, is also wholly barren of Graptolites. Immediately above 

 the Coniston Limestone, however, occurs a mass of dark mudstones, 

 of variable but never great thickness, to which the name of " Grapto- 

 litic Mudstones " was applied by Professor Harkness and myself, 

 owing to the extraordinary abundance in them of these organisms. 

 It is perfectly clear, therefore, that the seas of the area of the Lake- 

 district became abundantly stocked with Graptolites at a time imme- 



