146 



PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The differences, indeed, between the Graptolitic faunas of the Skid- 

 daw and Quebec regions are not greater than what may be due to 

 imperfect observation, or the effect of difference of station, without 

 either supposing that one region was peopled by migration from 

 the other*. 



The second area discussed by Dr. Nicholson is that of Wales and 

 Scotland, containing the Llandeilo Graptolites. The auther believes 

 that the north of England was dry land during the early portion of 

 the Llandeilo period, and also that the close of the Skiddaw 

 (Arenig) period was signalized by the upheaval of the Lake district ; 

 this, or the unfitness for life of the succeeding seas, in conse- 

 quence of igneous activity, may have been the cause of no deposits 

 containing Graptolites being superimposed upon the Skiddaw Slates, 

 until we reach the higher or later portion of the Caradoc or Bala 

 period (the Lower Llandovery), at which time an immigration of 

 Graptolites must have taken place from neighbouring seas. On such 

 hypotheses, supported by details too minute to discuss here, we may 

 realize the absence in certain areas of zoological groups whose 

 existence necessitates the deposition of sedimentary matter con- 

 genial to their growth and development. Why the Lower and 

 Upper Llandovery rocks of North and South Wales do not possess a 

 larger Graptolitic fauna, when in the north of England and Scotland 

 such a fauna is well developed, can only be explained through 

 physical changes, either movements of land and sea, or interference 

 through sedimentary agency, of which there may be many kinds. 



Dr. Nicholson further enters into the consideration of the second 

 great Graptolitic period of the north of England, during which the 

 mudstones of the Coniston series were deposited. In the Coniston 

 Limestone proper, which corresponds to the Bala Limestone, no 

 species have occurred, it being wholly barren of Hydrozoa; but the 

 so-called Graptolitic mudstones succeeding it abound in species, 6 

 genera and 25 species having been recorded from them. Many of 

 these (16) Nicholson believes have been derived from the Upper 

 Llandeilo of the south of Scotland, the remaining 9 being peculiar 

 to the Coniston Mudstones. The fourth area Dr. Nicholson calls the 

 Gala area of the south of Scotland^ in Dumfriesshire ; Mr. Lapworth 

 applied to these deposits the name " Gala Group." Eight species in 

 this Gala series are derived from the Upper Llandeilo of the Scotch 

 area, and the remaining 5 (of the 13) are importations from the 

 Coniston Mudstones and Coniston area of the Lake district. 



The Hudson-River shales and Utica Slates Nicholson believes to 

 have been peopled by a great migration of the Upper Llandeilo Grap- 

 tolites of the south of Scotland, which appear to have taken a 

 westerly course, and ultimately to have reached the United States, 

 there forming the Graptolitic fauna occurring in the Caradoc or 

 Hudson-River shales and Utica Slates described by James Hall. 

 This view of Nicholson's, that the Graptolites of the Hudson-River 

 group are derived through migration from the Upper Llandeilo of the 

 south of Scotland, he supports or illustrates by means of a table of 

 * Nicholson, Quart. Journ. Geol. Soc. vol. xxviii. pp. 218, 219. 



