429 - DYNAMICAL GEOLOGY. —  -[Boox TIL. 

temperature, and therefore of density, must occasion movements in ~ 
the mass of the oceanic waters.’ oe 
Apart from disputed questions in physics, the main facts for the — 
geological reader to grasp are—that a system of circulation exists in — 
the ocean; that warm currents move round the equatorial regions, 
and are turned now to the one side, now to the other, by the form of 
the continents along aud round which they sweep; that cold 
currents set in from poles to equator; and that, apart from actual 
currents, there is an extremely slow “creep” of the polar water 
under the warmer upper layers to the equator. 
(3.) Waves and Ground-Swell.-—A gentle breeze curls into ripples — 
the surface of water over which it blows. A strong gale or furious 
storm raises the surface into waves. The agitation of the water in a 
storm is prolonged to a great distance beyond the area of the 
original disturbance, and then takes the form of the long heaving ~ 
undulations termed ground-swell. Waves which break upon the 
land are called breakers, and the same name is applied to the 
ground-swell as it bursts into foam and spray upon the rocks. The 
concussion of earthquakes sometimes gives rise to very disastrous 
ocean waves (p. 272). 
The height and force of waves depend upon the breadth and — 
depth of sea over which the wind has driven them, and the form and 
direction of the coast-line. ‘The longer the “ fetch,” and the deeper 
the water, the higher the waves. A coast directly facing the 
prevalent wind will have larger waves than a neighbouring shore 
which presents itself at an angle to this wind or bends round so as — 
to form a lee-shore. The highest waves in the narrow British seas 
probably never exceed 15 or 20 feet, and usually fall short of that 
amount. The greatest height observed by Scoresby among the 
Atlantic waves was 43 feet.? 
Ground-swell propagated across a broad and deep ocean produces 
by far the most imposing breakers. So long as the water remains 
deep and no wind blows, the only trace of the passing ground-swell 
on the open sea is the huge broad heaving of the surface. But 
where the water shallows, the superficial part of the swell, travelling 
faster than the lower which encounters the friction of the bottom, 
begins to curl and crest as a huge billow or wall of water, that 
finally bursts against the shore. Such billows, even when no wind 
is blowing, often cover the cliffs of the north of Scotland with sheets 
of water and foam up to heights of 100 or even nearly 200 feet. 
During north-westerly gales, however, the windows of the Dunnet 
Head lighthouse, at a height of upwards of 300 feet above high- 
* The student may consult Maury’s “ Physical Geography of the Sea,” but more par- 
ticularly Dr. Carpenter’s papers in the Proceedings of the Royal Society for 1869-73, and 
Journal of Royal Geographical Society for 1871-77, on the side of temperature; and 
Herschel’s “ Physical Geography,” and Croll’s “ Climate and Time,” on the side of the 
winds, 
* Brit. Assoc. Rep. 1850, p. 26. A table of the observed heights of waves round 
Great Britain is given in Mr. T. Stevenson’s treatise on “‘ Harbours,” p. 20. 
