PORTLAND, ISLE OF, properly a peninsula of the coast of Dorsetshire, England, as a prolongation of a narrow ridge of shingle, Chesil Bank (q.v.), connects it with the mainland. Pop. (1901), 15,262. It is 4 m. long and nearly 1¾ in extreme breadth, with an area of about 4½ sq. m. The shores are wild and precipitous, and Portland is inaccessible from the sea except towards the south. The highest point, close upon 500 ft., is the Verne hill in the north. Wave action is seen in the numerous caverns, and south-east of Portland Bill, the southern extremity of the isle, is a bank called the Shambles, between which and the land there fiows a dangerous current called the Race of Portland. A raised beach is seen at Portland Bill. The substratum of the island is Kimeridge Clay, above which rests beds of sand and strata of Oolitic limestone, widely famed as a building stone. Extensive quarries, which are Crown property, have supplied the materials for St Paul's Cathedral and many other important public buildings. In the “ dirt-bed ” resting upon the Oolitic strata numerous specimens of petrified wood are found, some of great size. The soil, though shallow, is fertile, and mutton fed on the grass has a peculiar rich flavour. Quarrying, fishing and agriculture are the chief industries. Several curious local customs are retained by the inhabitants. A joint railway of the Great Western and London & South Western companies runs south from Weymouth to Portland (4¾ m.) and Easton (8½ m.) on the isle. The isle contains a convict prison with accommodation for about 1500 prisoners. Portland Castle, built by Henry VIII. in 1520, is generally occupied by the commander of the engineers or of the regiment stationed on the island. On a rock on the eastern side are remains of a more ancient fortress, Bow and Arrow Castle, ascribed to William Rufus.
A harbour of refuge, begun in 1847 under the direction of the Admiralty, was completed some fifteen years later. A breakwater stretching in a northerly direction from the north-east corner of the island partially enclosed a large area of water naturally sheltered on the south and west. An inner arm ran nearly east from the island and terminated in a masonry head and fort, and an outer detached arm bent to the north and terminated in a circular fort, a narrow entrance for shipping being left between the two. It was formed of a rubble mound quarried by convict labour at the summit of the island, and was lowered by a wire-rope incline to the sea. The harbour thus made was open on the north to Weymouth and the Channel, but the necessity for greater protection from torpedo attack made it advisable to complete the enclosure. Accordingly the Naval Works Acts of 1895 and subsequent years sanctioned works for closing the gap—about 2 m. long—between the end of the outer breakwater and the Bincleaves rocks near Weymouth, by two new breakwaters. One of these runs nearly east from the Bincleaves shore and is about 4642 ft. long, while from its extremity the other, about 4465 ft. long, stretches in a south-east direction towards the old outer breakwater, passages for navigation about 700 ft. wide separating it from its neighbours at each end. These new structures also consist of rubble mounds. The defensive harbour thus completely enclosed has an area of 2200 acres to the one-fathom line, of which 1500 acres have a depth of not less than 30 ft. at low water. There is no dockyard at Portland, but the watering and coaling arrangements for the supply of the fleet are of considerable importance. There is a coaling jetty and camber for the storage of both sea-borne and land-borne coal, with hydraulic appliances for handling it. The harbour and island are strongly fortified.
The isle of Portland is not mentioned in the time of the Romans. In 837 it was the scene of an action against the Danes, and in 1052 it was plundered by Earl Godwine. In 1643 the parliamentary party made themselves masters of the island and castle, but shortly afterwards these were regained by the Royalists through a clever stratagem, and not recovered again by the forces of the parliament till 1646.