490 Scientific Proceedings, Royal Dublin Society. 
valley of Scotland,* and the southern is the western continuation 
of the rocks in Wales and §.W. England.t The northern basin 
extends from Scotland into the province of Ulster, and from that, 
through Connaught, into the Atlantic. In N.E, Ulster, on account 
of the covering of Mesozoic and Cainozoic rocks, the Silurians 
only appear in one place, as conglomerates lying on Metamorphic 
rocks (Cambro-Silurians or Cambrians), near Cushendun, county 
Antrim; but in S.W. Ulster and in the adjoining portions of 
Connaught they appear more extensively. Here they are princi- 
pally of “Lower Old Red Sandstone” types; but near Drum- 
shambo, county Leitrim, and on the N.W. of Ballaghaderreen, 
county Mayo, there are green rocks of “ Upper Silurian” types ; 
while in the latter locality fossils not only of English “ Upper 
Silurian” species, but also of “ Lower Silurian” species, have been 
found. Farther westward—north, south, east, and west of Clew 
Bay—there occurs a detached portion of these Silurians, all of the 
“ Lower Old Red Sandstone” type. In this portion of the basin 
of deposit no sudden change has been observed ; for the most 
part the rocks are principally arenaceous, often conglomeratic ; the 
argillaceous rocks predominating to the west, in Clare Island and 
Louisburgh, west and south of Clew Bay. The rocks near Drum- 
shambo and N.W. of Ballaghaderreen are a little more argillaceous 
than those with which they are associated; but the principal 
difference between them is in their colour. 
But south of the western portion of these rocks, in the country 
north and south of ,Killary Harbour, and extending eastward to 
Loughs Mask and Corrib, rapid and remarkable changes take 
place in the accumulations, as here is found not only an inter- 
mingling of the rocks of the two types, but also zones containing 
fossil species characteristic of the Caradoc-Bala or English “ Lower 
Silurian”; while the characteristic fossils of the highest group of 
strata (Salrock Slates) is pronounced by Davidson to be of an 
Upper Llandovery type, although in the rocks below it they are 
of Wenlock and Ludlow species. 
Tf an attempt is made to calculate the age of these rocks palee- 
ontologically, according to the laws laid down in England, we 
are immediately puzzled, as the Irish paleontological evidence is 
* Geikie’s “ Lake Caledonia Basin.” t Geikie’s “‘ Welsh Lake Basin,” 
