Portland, ISLE OF, a rocky peninsula of Dorsetshire, connected with the mainland by the Chesil Bank (q.v.), and 4 miles S. of Weymouth by a branch-line (1865). It is miles long, wide, 9 in circumference, and 2890 acres in area. From its highest point, the Verne (495 feet), it shelves with a gradual and almost unbroken slope to Portland Bill (20 feet), the southern extremity, where stand two lighthouses (1716-89), showing fixed lights 210 and 136 feet above sea-level, and between which and the Shambles, a dangerous reef, 3 miles south-east, a surf, called the Portland Race, is raised by the rushing of the impetuous tides. The cliffs have in places been worn into fantastic caverns; and ancient raised beaches are well marked near the Bill. Portland is one solid mass of oolitic limestone, which has been largely quarried for building purposes since the 17th century, when Inigo Jones employed it for Whitehall and Sir Christopher Wren for St Paul's. Goldsmiths' Hall, the Reform Club, and Pall Mall generally are also built of it; and the yearly export now ranges between 50,000 and 70,000 tons.
There are three different qualities of Portland stone, the three strata lying close together. The top bed, called Roach, is unsuited for fine hewn work, since it is full of fossils, but it is hard and durable, and does well for the walls of docks, foundations, and the like. The Whit Bed, which comes next, yields the best stone for fine buildings. It varies in texture from a fine close grain to the roe-like structure characteristic of oolitic limestones, and is free from shells. Its colour is a pleasing grayish white. The Base Bed, not quite so much quarried as the others, is of finer grain and whiter than the Whit bed; but it is softer and better suited for internal than external architectural work. An analysis of this stone by Professor Daniell shows the following composition: Silica, 1.20; carbonate of lime, 95.16; carbonate of magnesia, 1.20; iron and alumina, .50; water and loss, 1.94; besides which ingredients there is often a trace of bitumen present. Portland stone is also quarried in the 'Isle' of Purbeck and the Vale of Wardour.
The formation of a magnificent harbour of refuge has been described at BREAKWATER, where also a map is given; most formidable fortifications have moreover been constructed, the Verne in especial being crowned by Fort Victoria. Other features of the 'Isle' are its great convict-prison, dating from 1848, and holding upwards of 1500 convicts (see PRISONS); Portland Castle (1520), built by Henry VIII., and held for Charles I. till 1646; Bow and
Arrow Castle, ascribed to Rufus; and Pennsylvania Castle (1800), built by Governor Penn, the great Quaker's grandson. The inhabitants of the 'Isle' long remained a peculiar people, intermarrying, and preserving, generation after generation, the many curious customs of their forefathers. The 'Isle' itself is remarkable for its copious and excellent spring-water and for its small breed of black-faced sheep, whose flesh, well known as 'Portland mutton,' is much esteemed for its flavour. Pop. (1851) 5195; (1881) 10,061; (1891) 9541.
See Damon's Geology of Weymouth and Portland (1860), and an article in the Cornhill (1882).