ABSTRACT The peculiar discoloration of some portions of the frozen ocean, differing in a remarkable degree from the ordinary blue or light green usual in other portions of the same sea, and quite independent of any optical delusion occasioned by light or shade, clouds, depth or shallowness, or the nature of the bottom, has, from a remote period, excited the curiosity or remark of the early navigators and whalemen, and to this day is equally a subject of interest to the visitor of these little-frequented parts of the world. The eminent seaman, divine, and savant, William Scoresby, was the first who pointedly drew attention to the subject, but long before his day the quaint old searchers after a North-west Passage “to Cathay and Zipango “seem to have observed the same phenomenon, and have recorded their observations, brief enough it must be acknowledged, in the pages of ‘Purchas—His Pilgrimes.’ Thus, Henry Hudson, in 1607, notices the change in the colour of the sea, but has fallen into error when he attributes it to the presence or absence of ice, whether the sea was blue or green—mere accidental coincidences. John Davis, when, at even an earlier date, he made that famous voyage of his with the “Sunshine “and the “Moonshine,” notes that, in the strait which now bears his name, “the water was black and stinking, like unto a standing pool.” More modern voyagers have equally noted the phenomenon, but without giving any-explanation, and it is the object of this paper to endeavour to fill up that blank in the physical geography of the sea. In the year 1860 I made a voyage to the seas in the vicinity of Spitzbergen and the dreary island of Jan Mayen, and subsequently a much more extended one through Davis’ Straits to the head of Baffin’s Bay, and along the shores of the Arctic regions lying on the western side of the former gulf, during which I had abundant opportunities of observing the nature of this discoloration. At that period I arrived at the conclusions which I am now about to state. In the course of the past summer I again made an expedition to Greenland, passing several weeks on the outward and homeward passages in portions of the seas mentioned, during which time I had an opportunity of confirming the observations I had made seven years previously, so that I consider that I am justified in bringing my researches, so far as they have, gone, before the Botanical Society.