compact, or in places very closely resembling chalk (Upway, Portisham, Brill, Chilmark). Nodules and layers of chert are well developed in some of the limestones of Dorsetshire and elsewhere; and a silicified oolite occurs near St Alban's Head. About Swindon, beds of sand are common in the Upper Portland beds with layers of calcareous sandstone (Swindon stone). Marly and sandy beds occur also at Shotover Hill. The lower portion is usually sandy and shows a gradual passage into the underlying Kimeridge Clay. W. H. Fitton in 1827 gave the name “ Portland Sand ” to this division. The Upper Portlandian in Dorsetshire is 130–170 ft. thick; the Lower Portlandian in the same district is 100–120 ft. These rocks crop out from South Dorsetshire into Wiltshire, Oxfordshire and Buckinghamshire, and possibly extend beneath younger rocks into Bedfordshire and Cambridgeshire. They have been proved by borings in Sussex and Kent, and in Yorkshire they are represented by part of the Speeton Clays, and in Lincolnshire by part of the Spilsby Sand. At Swindon and Aylesbury a conglomeratic layer with small pebbles of lydite and phosphatized fossils lies at the base of the Portland Stone.
The Upper Portlandian of England is characterized by the ammonite Perisphinctes giganteus, along with Cytheria (Cyrena) rugosa, Trigonia gibbosa, Perisphinctes boloniensis and Trigonia incurva as subzonal forms. Olcostephanus gigs is the zonal ammonite in the Lower Portlandian, associated with Trigonia Pellati, Cyprina Brongniarti, Exogyra brantrutana and Astarte Saemanni as subzonal indices. Other characteristic fossils are Cerithium portlandicum, the casts of which form the familiar “ Portland screw," Isastrae oblonga, the Chelonian Stegochelys; the remains of saurians Pliosaurus and Cimoliosaurus and others are found; Mesodon, Ischyodus and other fishes occur in this formation. The Portland limestones have been much in demand for building purposes; at Portland the “ Top Roach,” the “ Whit Bed ” or top freestone, and the “Best Bed ” (or Base Bed) are the best known. In the Vale of Wardour the lower Portlandian has been largely quarried; the stone from this neighbourhood is often described as Wardour, Tisbury or Chilmark stone. Swindon stone is a calcareous sandstone that occurs in the sands of the Upper Portland beds near Swindon.
Rocks of Portlandian age are well developed on the continent of Europe, but the grouping of the strata is different in some respects from that adopted by English geologists. In France the “ Portlandian ” is usually taken to include the Purbeckian as well as the equivalents of the English Portland beds, and some authors, e.g. E. Renevier, have included more or less of the Kimeridgian in this division. The Portlandian of north-west Germany includes the Eimbeckhäuser Plattenkalk and the Lower Portland Kalk. Oppel's “ Tithonian " (tithonic) division, embracing Upper Kimeridge beds, Portlandian and Purbeckian beds in the Alpine district, is now recognized as a deeper water deposit of this time with many points of resemblance to the Russian development to which the name “ Volgian " has been applied by S. Nikitin. The Portlandian beds of Yorkshire are more nearly related to the Volgian phase than to the beds of the same age in the south of England. The term Bononian (= Bolonian) was suggested by J. F. Blake in 1881 for a part of the Portlandian series, from their occurrence at Boulogne (Bononia) where they are similar to the beds of Dorset. He limited the name Portlandian to the Purbeckian and Upper Portlandian (Portland stone), while he placed the Portland Sands and upper part of 'the Kimeridge Clay in his Bolonian division: this scheme has not been accepted in England. See Jurassic.
PORTLOCK, JOSEPH ELLISON (1794–1864), British geologist
and soldier, the only son of Nathaniel Portlock, captain in the
Royal Navy, was born at Gosport on the 30th of September
1794. Educated at the Royal Military Academy he entered
the Royal Engineers in 1813. In 1814 he took part in the
frontier operations in Canada. In 1824 he was selected
by Colonel (afterwards Major-General) T. F. Colby (1784–1852)
to take part in Ordnance Survey of Ireland. He was engaged
for several years in the trigonometrical branch, and subsequently
compiled information on the physical aspects, geology
and economic products of Ireland. In 1837 he formed at Belfast
a geological and statistical office, a museum for geological
and zoological specimens, and a laboratory for the examination
of soils. The work was then carried on by Portlock as
the geological branch of the Ordnance Survey of Ireland, and
the chief results were embodied in his Report on the Geology of
the County of Londonderry and of parts of Tyrone and Fermanagh
(1843), an elaborate and well-illustrated volume in which he was
assisted by Thomas Oldham. After serving in Corfu and at
Portsmouth he was, in 1849, appointed Commanding Royal
Engineer at Cork, and from 1851–1856 he was Inspector of
Studies at the Royal Military Academy, Woolwich. For a
short time commanding officer at Dover, when the Council of
Military Education was formed in 1857 he was selected as a
member.
During these years of active service he contributed numerous geological papers to the scientific societies of Dublin and to the British Association. He published in 1848 a useful treatise on geology in Weale's “ Rudimentary Series ” (3rd. ed., 1853). He was president of the geological section of the British Association at Belfast (1852), and of the Geological Society of London (1856–1858). He wrote a Memoir of the late Major-General Colby, with a Sketch of the Origin and Progress of the Trigonometrical Survey (reprinted in 1869 from Papers on Subjects connected with the Royal Engineers, vols. iii.–v.). He also contributed several articles on military subjects to the 8th edition of the Encyclopaedia Britannica. He was elected a Fellow of the Royal Society in 1837. He died in Dublin on the 14th of February 1864.
PORT MAHON, or Mahon (Spanish Puerto Mahón), the
capital and principal seaport of Minorca, in the Spanish
province of the Balearic Islands. Pop. (1900), 17,144. Port
Mahon is situated on the east coast, at the head of a deep inlet
which extends inland for 3½ m. It is an important harbor
(see Minorca). The city occupies a conspicuous hill, and
presents a fine appearance from the sea; it is solidly built of
excellent stone. Many of the houses date from the British
occupation, which has also left curious traces in the customs
and speech of the people. The King's Island (Isla del Rey,
so called as the landing-place of Alphonso III. of Aragon in
1287) contains a hospital built by the admiral of the British
squadron in 1722; farther south-east on the shore is the village
of Villa Carlos or George Town, with ruins of extensive British
barracks; and at the mouth of the port, on the same side, are
the remains of Forte San Felipe, originally erected by Charles V.
and twice the scene of the capitulation of British troops. Opposite
San Felipe is the easily defended peninsula of La Mola
(256 ft. high), which is occupied by extensive Spanish fortifications.
Mahon is one of the principal quarantine stations of
Spain; the lazaretto, erected between 1798 and 1803, stands on
a long tongue of land, separated from La Mola by the inlet of
Cala Taulera. The principal modern buildings are the military
and naval hospitals, the theatre, museum, library and schools.
There are an arsenal and extensive quays. From its position
on the route of vessels plying between Algeria and the south of
France, the harbour is much frequented by French cargo-steamers;
it is also a Spanish naval station. The principal
exports are grain, live stock and fruit; cement, coal, iron,
machinery, flour, raw cotton and hides are imported. Shoes
and cotton and woollen goods are manufactured. About 250
vessels enter the port every year, and the annual value of the
foreign trade is, approximately, £200,000 to £250,000.
Mahon is the ancient Portus Magonis, which under the Romans was a municipium (Mun. flaviurn magontanum), probably including the whole island under its authority. As the name suggests, it had previously been a Carthaginian settlement. The Moors, who occupied Minorca in the 8th century, were expelled by James I. of Aragon in 1232. Khair-ed-Din Barbarossa besieged and captured the city in 1535; and in 1558 it was sacked by a corsair called Piali. The British, who under James Stanhope, afterwards Earl Stanhope, seized the island in 1708, made Mahon a flourishing city, and in 1718 declared it a free port. In 1756 it fell into the hands of the French through the failure of Admiral Byng to relieve the garrison of St Philip's (San Felipe). Restored to the British in 1762, it was in 1782 heroically but unsuccessfully defended by General Murray. In 1802 it was finally ceded to Spain by the treaty of Amiens.
PORTMANTEAU, a leather case or trunk for carrying articles
of personal use when travelling. The typical portmanteau of
