BRITISH ASSOCIATION. 
221 
The erratic block deposits of Ireland were distributed by a 
force moving generally in a north-west and south-east direction, 
modified, however, in many localities by the opposition of moun- 
tain ridges and deep valleys. This deposit consists usually of 
clay and rolled fragments of limestone, derived from the car- 
boniferous rocks, which occupy two-thirds of the island; and 
these materials seldom exhibit any trace of stratification. It 
frequently exceeds one hundred feet in thickness, and occupies 
all the valleys, and even the interior of the mountains, occurring 
either in one thick mass, or in elongated ridges, known by the 
name of Eskers. The prevailing direction of the mountain ridges 
of Ireland is north-east and south-west, that is, at right angles 
to the supposed direction of the current by which the drift was 
deposited ; in consequence of this, the deposits of detrital lime- 
stone are distributed only on the north-western declivities of the 
hills. In the granite and slate hills of Wicklow, the limestone 
gravel has passed through the valleys of that range, and formed 
a valuable deposit to the east of Mount Leinster, spreading over 
the Cambrian slatse, east of Newtown Barry, in the county of 
Wexford ; the same facts may be observed in the Slievebloom 
mountains, in the Galtees, and the Monavullagh mountains. 
The summit of Blatinglass Hill, on the northern edge of the 
Wicklow granite district, is 1256 feet above the level of the sea; 
to the eastward of this hill there is no limestone gravel, whilst it 
is abundant on the lower hills, and in the valleys to the south ; 
to the north of this hiM the gravel is seen at an elevation of 880 
feet, from which Mr. Grifiith infers that the transporting force 
did not operate at an elevation exceeding 1000 feet. Besides 
the limestone gravel, boulders of granite and conglomerate are 
occasionally found scattered on the surface, and not included in 
the clay drift ; from the peculiar composition of the granite of 
certain districts, it is not difiicult to detect the locality from 
which these blocks were derived, and hence the author has been 
enabled to detect small boulders of the Connemara granite, in 
