_April 9th._—Yesterday Mr. Agassiz lectured upon the traces of glaciers as they exist in the northern hemisphere, and the signs of the same kind to be sought for in Brazil. After a sketch of what has been done in glacial investigation in Europe and the United States, showing the great extension of ice over these regions in ancient times, he continued as follows: “When the polar half of both hemispheres was covered by such an ice shroud, the climate of the whole earth must have been different from what it is now. The limits of the ancient glaciers give us some estimate of this difference, though of course only an approximate one. A degree of temperature in the annual average of any given locality corresponds to a degree of latitude; that is, a degree of temperature is lost for every degree of latitude as we travel northward, or gained for every degree of latitude as we travel southward. In our times, the line at which the average annual temperature is 32°, that is, at which glaciers may be formed, is in latitude 60° or thereabouts, the latitude of Greenland; while the height at which they may originate in latitude 45° is about 6,000 feet. If it appear that the ancient southern limit of glaciers is in latitude 36°, we must admit that in those days the present climate of Greenland extended to that line. Such a change of climate with reference to latitude must have been attended by a corresponding change of climate with reference to altitude. Three degrees of temperature correspond to about one thousand feet of altitude. If, therefore, it is found that the ancient limit of glacier action descends on the Andes, for instance, to 7,000 feet above the level of the sea under the equator, the present line of perpetual snow being at 15,000, it is safe to infer that in those days the climate was some 24° or thereabouts below its present temperature. That is, the temperature of the present snow line then prevailed at a height of 7,000 feet above the sea level, as the present average temperature of Greenland then prevailed in latitude 36°. I am as confident that we shall find these indications at about the limit I have pointed out as if I had already seen them. I would even venture to prophesy that the first moraines in the valley of the Marañon should be found where it bends eastward above Jaen.”[9]