Paleo-Geological and Geographical Maps of the British Islands. 265 
forms. In the Bohemian basin, as M. Barrande* has shown, there was a pro- 
digious development of life ; but often, though many hundreds of feet of slates and 
erits in some districts, no trace of organic structure 1s discoverable. 
Puate XXV. 
Upper Silurian and Devono-Silurian Periods, 
The relative areas of land and sea during these periods differ widely from those of 
the period which preceded it, as will be seen on a comparison of Plates XXTV. and 
XXV. The sea which overspread the whole of the British Isles and adjoming 
portions of France and Belgium is now restricted mainly to the southern and central 
portions of the British Isles ; while large tracts in the north and west, as well as 
Normandy and Brittany, are converted into land surfaces, holding in their deep 
depressions, lakes or fresh-water basins, which were formed towards the close of 
the Silurian period, or in more definite terms, during the Devono-Silurian stage. 
Nature of the Upper Silurian Beds.—The basement beds of the Upper Silurian 
series (Llandovery beds) are frequently conglomerates and sandstones, derived 
from the disintegration of the rocks of the adjoining lands, and by their position 
we are able to indicate the position of the margins of these lands themselves. Such 
is the case in the districts of Connemara in the county Galway,t and of Builth in 
Radnorshire.t The succeeding beds consist of grits, shales and limestones of the 
Wenlock and Ludlow series, often rich in marine fossils. These beds occur in 
West Galway and Mayo, in North and South Wales, and Monmouthshire, in 
Staffordshire, along the southern slopes of the Cumberland mountains, and those 
of the southern uplands of Scotland. In the north of France and Belgium they 
are altogether wanting, as the Devonian beds rest against the shelving flanks of 
ancient lands formed of Lower Silurian and Cambrian strata.§ 
Devono-Silurian Beds.—Under this term I include a series of beds known by 
various names, and chiefly developed to the north and to the south of the British 
area. They include the “ Passage beds” of Murchison, and the “ Downton sand- 
stone,” lying at the top of the Upper Ludlow rock, in South Wales ; the “ Dingle 
and Glengariff Beds” of Jukes, forming the south-western mountains of Jreland, 
and seen resting conformably on the Upper Silurian beds along the coast of Dingle : 
“the Fintona beds” of the north of Ireland, which rest unconformably on older 
crystalline strata ; and the “ Lower Old Red Sandstone ” of Scotland. The Devono- 
Silurian beds form the connecting series between the Upper Silurian and the 
estuarine Devonian beds of Monmouth and South Wales, and are probably re- 
* «Syst. Sil. de la Bohéme.” +“ Phys. Geol. of Ireland,” p. 23. 
+ Ramsay, “ Phys. Geol. of England and Wales,” p. 89. § Mourlon, “ Géol. de la Belgique,” t. 1, p. 54. 
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