BOSPHORUS (or, more correctly, Bosporus, from the Greek Βόσπορος, Ox ford), originally used for a strait, was especially applied to the Bosporus Cimmerius, or Strait of Yenikale, and the Bosporus Thracius, or Strait of Constantinople. In modern times it has almost become the exclusive designation of the latter strait, which unites the Black Sea with the Sea of Marmora and forms the boundary between Europe and Asia. The channel is about 16 English miles in length, and has a maximum breadth of nearly 2 miles, a minimum breadth of 550 yards, and along the middle a varying depth of 148 to 388 feet. In the centre there is a rapid current from the Black Sea to the Sea of Marmora, and a counter-current sets in along each shore. At the narrows the three currents produce a most violent commotion. The average temperature of the water hardly differs from that of the air, but it almost never reaches the atmospheric maxima and minima. The surface is very rarely frozen over, not more than five or six instances being recorded since the 8th century a.d. The Golden Horn was partially frozen over in 1849 and 1862. The shores of the Bosphorus are composed in the northern portion of different volcanic rocks, such as dolerite, granite, and trachyte; but along the remaining course of the channel the prevailing formations are Devonian, consisting of sandstones, marls, quartzose conglomerates, and calcareous deposits of various kinds. The scenery on both sides is of the most varied and beautiful description.