Isle Of Portland, a peninsula and parish of Dorsetshire, England, projecting into the English channel, 3 m. S. of Weymouth, about half way between Portsmouth and Plymouth, and 50 m. S. W. of Southampton; pop. of the parish in 1871, 9,907, including convicts, soldiers, and non-residents employed on government works. It is nearly 4 m. long by 1 to 1 1/2 m. broad, and 9 m. in circumference, varies in height from 458 ft. to less than 50 ft., and is surrounded on all sides by inaccessible cliffs, excepting on the north opposite Weymouth, which is the only landing place. It is still called an island, although it has been for centuries connected with the mainland at Abbotsbury by Chesil Bank, an irregular ridge of loose shingle 10 m. long and dangerous to navigation. At the S. extremity of the rocky hills is Portland Bill (anc. Vindelia Promontorium), a name occasionally given to the whole peninsula on account of its beak-like shape. On the north stands the ponderous Portland castle, erected by Henry VIII. about 1520, which defends the coast and is occupied by the lieutenant governor of Portland. The place is especially celebrated for a stupendous breakwater completed in 1872 (see Breaewater), and for its freestone, called Portland stone, consisting of three kinds; the lowest strata are the whitest and finest, and some of the stones weigh from 5 to 14 tons each.

St. Paul's cathedral and many other great structures in London have been built of this stone, of which about 40,000 tons are exported annually, and the quarries are connected by a railway with the pier. As many as 700,000 tons a year were at various periods used for the breakwater. Connected with the latter are a naval station, a harbor of refuge, and batteries. A penal settlement was established here in 1848. The prison is built on elevated ground in a place called "The Grove," and has eight wings, accommodating 1,500 convicts, besides a hospital, a chapel, barracks, and cottages for the wardens. Hundreds of the convicts were employed on the breakwater. The fisheries are extensive, especially of mackerel. The soil in the vicinity is exceedingly fertile, and sheep raised in large numbers yield the famous Portland mutton. In the neighborhood are Pennsylvania castle, built by a member of the Penn family, and the ruined Rufus or Bow and Arrow castle, built by King William Rufus. William and Henry Bentinck were respectively created earl and duke of Portland in 1689 and 1716.