Lake Ontario, the lowest and smallest of the chain of five great lakes of the northern United States and Canada. The name is Indian, meaning beautiful. The lake extends E. and W. about 180 m., having a mean breadth of 35 m., and a depth supposed to average about 500 ft. The elevation of its surface above tide being 231 ft., its bottom is about as far below the level of the ocean as its surface is above it. The area of the lake is computed at 6,300 sq. m., 3,300 sq. m. less than that of Lake Erie, the next larger lake. The boundary line between the United States and Canada runs through the central portion of Lake Ontario, from the mouth of the Niagara river to the outlet in the extreme N. E. corner. This is the St. Lawrence river, which, commencing at this point, pursues a course of nearly 800 m. to the gulf of St. Lawrence. From the head of the river the coast of Lake Ontario on its E. and S. sides as far as Niagara river belongs to the state of New York; thence 50 m. further W. along the S. coast, and E. along the N. side to the St. Lawrence river, the lake is bounded by Ontario, Canada. By reason of its great depth Lake Ontario is much less disturbed by storms than Lake Erie, and its navigation is also much less obstructed by ice.

In the severest winters the boats continue their trips across, and are rarely interrupted by ice. When once chilled, the water slowly recovers a warmer temperature; and even in the middle of May for two successive years, 1837 and 1838, it has been found that the temperature of the water a little below the surface in the central portion of the lake was only from 36° to 38°, while near the American shore it was from 52° to 68°, and at the same times at Cobourg on the Canadian side from 48° to 51°. Prof. C. Dewey, by whom these observations are recorded in the " American Journal of Science," supposed that the accumulation of ice in Lake Erie, which frequently does not disappear till some time in May, serves to retain the low temperature of the water, particularly along the course of the main current through the central part of Lake Ontario. The effect of this is to retard the approach of spring; but opposite causes operate in the autumn to check the advance of winter. The same observer has recorded the measures of the varying level of the lake from the year 1845 to 1859, made at the mouth of the Genesee by order of the government.

From these it appears that there is no periodical rise and fall, and the variations are dependent on very regular and adequate causes of supply and drain. The range of rise and fall is 54 inches, the maximum elevation during the years of the observation being in February and the minimum in August. The effect of long continued rains or of long droughts in certain years is observed in the occurrence of the highest or lowest water out of the usual seasons. - The country around Lake Ontario is in general fertile and well populated by agricultural, manufacturing, and commercial communities. On the N. side the surface rises gradually from the lake shore and spreads out in broad plains. From the St. Lawrence river two thirds of the way to the W. extremity of the lake these are underlaid by the lower Silurian limestones, from which the soil derives its fertility. These rocks near Toronto pass beneath the group of the Hudson river slates, and these then occupy the surface nearly to Burlington. The red shales and sandstones of the Medina group succeed, and a narrow belt of this formation borders the lake on its W. and S. sides. At Oswego the lower formations begin to reappear, and their outcrops are successively passed over along the E. extremity of the lake.

The formations which underlie the lake and form its bottom, over the N. half at least, are no doubt these lower limestones sloping southwardly from the N. shore. A marked feature in the topography of the S. shore is the "Lake ridge," a narrow elevation ranging from Sodus in Wayne co. to the Niagara river, nearly parallel with the edge of the lake, and at a distance of from 3 to 8 m. from it. Its elevation is in places nearly 200 ft. above the lake, and generally exceeds 160 ft. The surface on each side slopes away gradually, so that the line of the ridge is not everywhere distinctly defined. In other places it is plainly marked, having a base from 56 to 112 ft. across, and a width at the summit of about 33 ft. Sometimes it is divided into three or four parallel ridges, which extend a few rods and then unite in one. Being composed of sand and gravel, it makes one of the finest natural roads in the world, and the principal highway along this side of the lake has been upon its summit. There can be little doubt that this ridge was an ancient shore line, and that within a recent geological period it has been formed by the waters of the lake. - Besides its main feeder, the Niagara river, the principal streams which flow into Lake Ontario are the Genesee, Oswego, and Black rivers.

The Oswego is the outlet of almost all the lakes in the western part of New York. On the N. side of Lake Ontario a range of hills extending parallel with the lake and a few miles back from it throws the drainage generally in other directions. The Trent river alone finds a passage through these hills, and flows with a number of smaller streams into the bay of Quints, a long inlet extending about 70 m. between the peninsula of Prince Edward near the foot of the lake and the mainland. The largest island in the lake, called Amherst island, is at the mouth of this inlet; it is 10 m. long and 6 broad. Many other smaller islands are met with at this lower extremity of the lake; but excepting in this part the coast is very regular and unbroken. The principal towns in New York on the lake, or near the mouths of the streams which flow into it, are Lewiston on the Niagara river, Rochester on the Genesee, Oswego at the mouth of the Oswego river, and Sackett's Harbor near the foot of the lake. In Canada, Kingston at the foot of the lake, Toronto 35 m. from its head, and Hamilton at the extreme head, are the largest towns on its shore.

Its navigation is connected with that of Lake Erie by the Welland canal of Canada, 28 m. long.