The vast Laurentian area of the north-eastern part of North America forms a very considerable proportion of the whole continent and, geologically speaking, it may be regarded as its nucleus. Greenland, on the other side of Baffins’ Bay and Davis’ Straits, consists, as far as known, almost altogether of the same rocks. Hudson’s Bay, which is nearly half as extensive as the Mediterranean Sea, lies in the middle of the continental portion of the great Laurentian area and the waters drain into it from all sides. The country slopes towards it from the Rocky Mountains, more than 1,300 miles to the westward, from the centre of Labrador, 500 miles to the eastward and from the immediate vicinity of Lake Superior in the south. Thus, the waters of this great interior sea occupy only the centre of what may be called the basin of Hudson’s Bay. This wide depression has existed from early geological times, as is shown by the Manitounuck rocks of the eastern shore and islands and by the Silurian limestones of Mansfield and Southampton Islands and of the southwestern shores of Hudson’s Bay and James’ Bay, as well as the Devonian rocks of the latter. These flat-lying palæozoic strata probably also extend over much of the bed of the bay, judging by its shallowness and the uniform depth of its waters, and also from the composition of the materials of the drift which have been carried out of it during the glacial period. None of the unaltered rocks around the Bay have undergone any marked disturbance, so far as known. The observations of Dr. Franz Boas and others around Fox Channel would lead us to suppose that from a geological standpoint this body of water is a repetition of Hudson’s Bay on a smaller scale (see above). [Sidenote: Source of volcanic rocks.] [Sidenote: Clearwater Lake.]