230 PRINCIPLES OF STRATIGRAPHY 



strengthened. Between the cliffs of the Pentland fjord the tidal 

 stream known as the Roost has a velocity of lo to ii knots, and 

 even steamers which have a speed of more than 1 1 knots avoid 

 steaming against this stream when the wind favors it. Between the 

 Orkneys and the north coast of Scotland, tidal streams of 8 to lo 

 knots are normal during spring tides. In such tidal races irregu- 

 larities of the coast will produce whirlpools such as the one off 

 Mosken and Varo, among the Lofoten Islands, famous for centuries 

 as the "Maelstrom" ; or the even more dangerous "Saltstrom" at 

 the mouth of the Saltenfjord on the Norwegian mainland oppo- 

 site. The whirlpool known since the days of Homer as the 

 Charybdis, in the Straits of Messina, and the fainter but equally 

 noted maelstroms at Scilla, on the Italian coast of the Straits, owe 

 their peculiarities to the meeting in the narrow passage of the tides 

 from the Tyrrhenian and Ionian seas, which have a phase differ- 

 ence of 6 hours, and thus high water approaches from one and low 

 water from the other side, the differences being adjusted by the 

 strong currents generated which change in direction every six 

 hours. Owing to the conformation of the borders and bottom of 

 the passage, strong whirls of water are produced, which bring the 

 colder and more saline waters from the deeper parts and with it 

 deep water organisms, such as the larval form of the eel, etc. 



Tidal currents extend to much greater depth than that reached 

 by wave motion. North of the Dogger bank, in water 73 meters 

 deep, the maximum tidal current in the upper 10 meters of water, 

 measured at intervals of six hours, was 15.9, 20.4 and 20.1 cm. per 

 second, and in 70 meters' depth or 3 meters above the bottom, the 

 corresponding velocities were 8.9, 13.2 and 10.2 cm. per second. 

 The tidal wave in this case was not over i meter high ; deep water 

 waves of this height would be imperceptible at a depth of 70 

 meters, while the tidal stream still retained 56, 65 and 51 per cent, 

 respectively, of its surface velocity. 



Tides and tidal streams are not as pronounced in the mediter- 

 raneans as in the open sea, and this difference becomes emphasized 

 when the outlet of the mediterranean is narrow, as in the case of 

 the Straits of Gibraltar. The Gulf of ^Mexico ( Mexican mediter- 

 ranean), with a wider opening, still illustrates this phenomenon, the 

 range of the tide at Galveston, Texas, being less than i foot. 



Tides independent of the Atlantic tides and more nearly com- 

 parable to the seiches in lake basins also occur. In still more 

 enclosed basins, as in the Black Sea, tides are wanting altogether. 

 In large lakes a periodic rise and fall of the water of slight extent 

 has been observed and compared with the tides. These lake tides 



