150 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



thus appears that when the main volcanic activity came to a close 

 it was succeeded by a time of such quiescence, and such slow, tran- 

 quil sedimentation in clear, perhaps moderately deep water, that a 

 true Radiolarian ooze gathered over the sea-bottom *. 



(d) Ieelajjd. — Llandeilo, Bala, Uppee-Silurian Periods. 



Into the east of Ireland the Lower-Silurian rocks are prolonged 

 from Scotland, from the Lake District, and from Wales. Though 

 greatlj'' concealed under younger formations across the breadth of the 

 island, and occasionally interrupted by the uprise of still older masses 

 than themselves, they nevertheless occupy by much the larger part 

 of the maritime counties from Belfast Lough to the southern coast- 

 line of "Waterford, even as far as Dungarvan Harbour. With the 

 same lithological tj-pes of sedimentary deposits as in other parts of 

 the United Kingdom, they carry with them here also their charac- 

 teristic records of contemporaneous volcanic action. Though no- 

 where piled into such magnificent mountain-masses as in Westmore- 

 land and I^orth Wales, they are traceable over a wider area than in 

 any other region of Britain. 



IMuch remains to be done, both in the field and in the laboratory 

 and microscope-room, before our knowledge of the vulcanism of 

 Lower-Silurian time in Ireland is brought abreast of our acquaint- 

 ance with that of other portions of the United Kingdom. In espe- 

 cial, the several geological horizons of the rocks have only been 

 approximately fixed. Great difSculty was experienced by the 

 Geological Survey in drawing any satisfactory line between Llandeilo 

 and Bala rocks. This arose not so much from deficiency of fossil 

 evidence as from the way in which the fossils of each group seemed 

 to occur in alternating bands in what might be regarded as a con- 

 tinuous series of strata. Indeed, in some localities it almost 

 appeared as if the occurrence of one or other fades of fossils de- 

 pended mainly on lithological characters indicative of original 

 conditions of deposit, for the Llandeilo forms recurred where black 

 shales set in, while Bala forms made their reappearance where calca- 

 reous and gritty strata predominated t. It remains to be seen how 

 far there is any foundation in actual fact for this impression. More 

 recent work in Scotland rather suggests that the parallel repetition 



^ Ann. Mag. Xat. Hist. (1890) 6th ser. vi. p. 40. 



t Jukes was disposed to regard the two faunas as essentially coeval, but 

 inhabiting different kinds of sea-bottom. See his note, Explanation of Sheets 

 167, 168, 178, 179, p. 30. 



