the
southern border of the county are the extremities of the great group
belonging to the adjacent county Wicklow. The principal summits are the
group containing Glendoo (1919 ft.) and Two Rock (1699 ft.) within the
county, and the border group of Kippure, reaching in that summit a
height of 2475 ft. The grandest features of these hills are the great
natural ravines which open in them, the most extraordinary being the
Scalp through which the traveller passes from Dublin to Wicklow.

_Geology._--On the north a Silurian upland stretches, falling to the
sea at Balbriggan, where fossiliferous strata contain contemporaneous
volcanic rocks. A limestone of Bala age comes out under shales and
andesites in the promontory of Portrane, and rocks of the same series
occur in the bold island of Lambay, associated with a large mass of
dark green porphyritic andesite (the "Lambay porphyry"). Silurian
rocks reappear at Tallaght in the south-west, where the granite of
Leinster rises through them, forming a moorland 2000 ft. in height
only a few miles south of Dublin. Old Red Sandstone, seen at Donabate
and Newcastle, leads up into Carboniferous Limestone, which is often
darkened by mud and even shaly ("calpy" type). This rock produces a
fairly level country, both north and south of the valley of the
Liffey, although the beds are greatly folded. Beds of a higher
Carboniferous zone are retained in synclinals near Rush. The rugged
peninsula of Howth, connected by a raised bench with the mainland, is
formed of old quartzites and shales, crushed and folded, and probably
of Cambrian age. The rocks of the county show many signs of
ice-action, and boulder-clays and drift-gravels cover the lowland, the
latter being banked up on the mountain-slopes to heights of 1200 ft.
or more. Much of this glacial material has been imported from the area
of the Irish Sea. Lead-ore has been mined at the granite-contact at
Ballycorus.

_Industries._--The extension of Dublin city and its suburbs has no
doubt had its influence on the decrease of acreage under both tillage
and pasture. Oats and potatoes are the principal crops, but live
stock, especially cattle, receives greater attention. A large
proportion of holdings are of the smallest, nearly one-half of those
beneath fifteen acres being also beneath one acre. The manufactures of
the county are mainly confined to the city and suburbs, but there is
ma