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 But he also found that dogs exposed, for moderate periods, to similar pressures suffered no ill effects provided that the pressure was relieved gradually, in 1-1 hours; and his results have been confirmed by subsequent investigators. To prevent caisson disease, therefore, the decompression should be slow; Leonard Hill suggests it should be at a rate of not less than 20 minutes for each atmosphere of pressure. Good ventilation of the caisson is also of great importance (though experiment does not entirely confirm the view that the presence of carbonic acid to an amount exceeding 1 or 1 parts per thousand exercises a specific influence on the production of compressed air illness), and long shifts should be avoided, because by fatigue the circulatory and respiratory organs are rendered less able to eliminate the absorbed gas. Another reason against long shifts, especially at high pressures, is that a high partial pressure of oxygen acts as a general protoplasmic poison. This circumstance also sets a limit to the pressures that can possibly be used in caissons and therefore to the depths at which they can be worked, though there is reason to think that the maximum pressure (4 atmospheres) so far used in caisson work might be considerably exceeded with safety, provided that proper precautions were observed in regard to slow decompression, the physique of the workmen, and the hours of labour. As to the remedy for the symptoms after they have appeared, satisfactory results have been obtained by replacing the sufferers in a compressed air chamber (“recompression”), when the gas is again dissolved by the body fluids, and then slowly “decompressing” them.

CAITHNESS, a county occupying the extreme north-east of Scotland, bounded W. and S. by Sutherlandshire, E. by the North Sea, and N. by the Pentland Firth. Its area is 446,017 acres, or nearly 697 sq. m. The surface generally is flat and tame, consisting for the most part of barren moors, almost destitute of trees. It presents a gradual slope from the north and east up to the heights in the south and west, where the chief mountains are Morven (2313 ft.), Scaraben (2054 ft.) and Maiden Pap (1587 ft.). The principal rivers are the Thurso (“Thor’s River”), which, rising in Cnoc Crom Uillt (1199 ft.) near the Sutherlandshire border, pursues a winding course till it reaches the sea in Thurso Bay; the Forss, which, emerging from Loch Shurrery, follows a generally northward direction and enters the sea at Crosskirk, a fine cascade about a mile from its mouth giving the river its name (fors, Scandinavian, “waterfall;” in English the form is force); and Wick Water, which, draining Loch Watten, flows into the sea at Wick. There are many other smaller streams well stocked with fish. Indeed, the county offers fine sport for rod and gun. The lochs are numerous, the largest being Loch Watten, 2 m. by m., and Loch Calder, 2 by 1 m., and Lochs Colam, Hempriggs, Heilen, Ruard, Scarmclate, St John’s, Toftingale and Wester. So much of the land is low-lying and boggy that there are no glens, except in the mountainous south-west, although towards the centre of the county are Strathmore and Strathbeg (the great and little valleys). Most of the coast-line is precipitous and inhospitable, particularly at the headlands of the Ord, Noss, Skirsa, Duncansbay, St John’s Point, Dunnet Head (346 ft.), the most northerly point of Scotland, Holburn and Brims Ness. From Berriedale at frequent intervals round the coast occur superb “stacks,” or detached pillars of red sandstone, which add much to the grandeur of the cliff scenery.

Caithness is separated from the Orkneys by the Pentland Firth, a strait about 14 miles long and from 6 to 8 miles broad. Owing to the rush of the tide, navigation is difficult, and, in rough weather, dangerous. The tidal wave races at a speed which varies from 6 to 12 m. an hour. At the meeting of the western and eastern currents the waves at times rise into the air like a waterspout, but the current does not always nor everywhere flow at a uniform rate, being broken up at places into eddies as perilous as itself. The breakers caused by the sunken reefs off Duncansbay Head create the Bores of Duncansbay, and eddies off St John’s Point are the origin of the Merry Men of Mey, while off the island of Stroma occurs the whirlpool of the Swalchie, and off the Orcadian Swona is the vortex of the Wells of Swona. Nevertheless, as the most direct road from Scandinavian ports to the Atlantic the Firth is used by at least 5000 vessels every year. In the eastern entrance to the Firth lies the group of islands known as the Pentland Skerries. They are four in number—Muckle Skerry, Little Skerry, Clettack Skerry and Louther Skerry—and the nearest is 4 m. from the mainland. On Muckle Skerry, the largest ( m. by m.), stands a lighthouse with twin towers, 100 ft. apart. The island of Stroma, 1 m. from the mainland (pop. 375), belongs to Caithness and is situated in the parish of Canisbay. It is 2 m. long by 1 m. broad. In 1862 a remarkable tide climbed the cliffs (200 ft.) and swept across the island.

Geology.—Along the western margin of the county from Reay on the north coast to the Scaraben Hills there is a narrow belt of country which is occupied by metamorphic rocks of the types found in the east of Sutherland. They consist chiefly of granulitic quartzose schists and felspathic gneisses, permeated in places by strings and veins of pegmatite. On the Scaraben Hills there is a prominent development of quartz-schists the age of which is still uncertain. These rocks are traversed by a mass of granite sometimes foliated, trending north and south, which is traceable from Reay southwards by Aultnabreac station to Kinbrace and Strath Helmsdale in Sutherland. Excellent sections of this rock, showing segregation veins, are exposed in the railway cuttings between Aultnabreac and Forsinard. A rock of special interest described by Professor Judd occurs on Achvarasdale Moor, near Loch Scye, and hence named Scyelite. It forms a small isolated boss, its relations to the surrounding rocks not being apparent. Under the microscope, the rock consists of biotite, hornblende, serpentinous pseudo-morphs after olivine and possibly after enstatite and magnetite, and may be described as a mica-hornblende-picrite. The remainder of the county is occupied by strata of Old Red Sandstone age, the greater portion being grouped with the Middle or Orcadian division of that system, and a small area on the promontory of Dunnet Head being provisionally placed in the upper division. By means of the fossil fishes, Dr Traquair has arranged the Caithness flagstone series in three groups, the Achanarras beds at the base, the Thurso flagstones in the middle, and the John o’ Groats beds at the top. In the extreme south of the county certain minor subdivisions appear which probably underlie the lowest fossiliferous beds containing the Achanarras fauna. These comprise (1) the coarse basement conglomerate, (2) dull chocolate-red sandstones, shales and clays around Braemore in the Berriedale Water, (3) the brecciated conglomerate largely composed of granite detritus seen at Badbea, (4) red sandstones, shales and conglomeratic bands found in the Berriedale Water and further northwards in the direction of Strathmore. Morven, the highest hill in Caithness, is formed of gently inclined sandstones and conglomerates resting on an eroded platform of quartz-schists and quartz-mica-granulites. The flagstones yielding the fishes of the lowest division of the Orcadian series appear on Achanarras Hill about three miles south of Halkirk. The members of the overlying Thurso group have a wide distribution as they extend along the shore on either side of Thurso and spread across the county by Castletown and Halkirk to Sinclairs Bay and Wick. They are thrown into folds which are traversed by faults some of which run in a north and south direction. They consist of dark grey and cream-coloured flagstones, sometimes thick-bedded with grey and blue shales and thin limestones and occasional intercalations of sandstone. In the north-west of the county the members of the Thurso group appear to overlap the Achanarras beds and to rest directly on the platform of crystalline schists. In the extreme north-east there is a passage upwards into the John o’ Groats group