This is a large rock, of a conical or sugarloaf shape, the summit or upper part of which is perfectly white, from the quantity of birds' dung, with which it is covered. Though it had been seen many times, its true situation was unknown till the year 1810, when it was ascertained by Mr. T. Harvey, master of the Endi/?nicn frigate. Together with the bank on which it stands, it was examined by Captain Vidal, R.N., in 1831, and again, in August, 1862, by the officers of H.M.S. Porcupine, when surveying the bank of deep soundings West of Ireland, and during which survey the Porcupine Bank was discovered. This solitary rock, the summit of a submarine mountain, emerges from the ocean in lat. 57° 36' 20" N., long. 13° 41' 30" W., 225 miles N. 53° W., true, from Tory Island, off the North coast of Ireland. It rises to an elevation of 70 ft. above the sea, is about 250 ft. in circumference at its base, and is composed of a coarse granite. The summit, sharp-pointe...