Second. To this succeeded a prolonged retreat of the great glaciers, when they evacuated not only the Jura and the low country between that chain and the Alps, but retired some way back into the Alpine valleys. M. Morlot supposes their diminution in volume to have accompanied a general subsidence of the country to the extent of at least 1000 feet. The geological formations of the second period consist of stratified masses of sand and gravel, called the "ancient alluvium" by MM. Necker and Favre, corresponding to the "older or lower diluvium" of some writers. Their origin is evidently due to the action of rivers, swollen by the melting of ice, by which the materials of parts of the old moraines were rearranged and stratified and left usually at considerable heights above the level of the present valley plains. Third. The glaciers again advanced and became of gigantic dimensions, though they fell far short of those of the first period. That of the Rhone, for example, did not again reach the Jura, though it filled the Lake of Geneva and formed enormous moraines on its borders and in many parts of the valley between the Alps and Jura. Fourth. A second retreat of the glaciers took place when they gradually shrank nearly into their present limits, accompanied by another accumulation of stratified gravels which form in many places a series of terraces above the level of the alluvial plains of the existing rivers. In the gorge of the Dranse, near Thonon, M. Morlot discovered no less than three of these glacial formations in direct superposition, namely, at the bottom of the section, a mass of compact till or boulder-clay (Number 1) 12 feet thick, including striated boulders of Alpine limestone, and covered by regularly stratified ancient alluvium (Number 2) 150 feet thick, made up of rounded pebbles in horizontal beds. This mass is in its turn overlaid by a second formation (Number 3) of unstratified boulder clay, with erratic blocks and striated pebbles, which constituted the left lateral moraine of the great glacier of the Rhone when it advanced for the second time to the Lake of Geneva. At a short distance from the above section terraces (Number 4) composed of stratified alluvium are seen at the heights of 20, 50, 100, and 150 feet above the Lake of Geneva, which by their position can be shown to be posterior in date to the upper boulder-clay and therefore belong to the fourth period, or that of the last retreat of the great glaciers. In the deposits of this fourth period the remains of the mammoth have been discovered, as at Morges, for example, on the Lake of Geneva. The conical delta of the Tiniere, mentioned in Chapter 2 as containing at different depths monuments of the Roman as well as of the antecedent bronze and stone ages, is the work of alluvial deposition going on when the terrace of 50 feet was in progress. This modern delta is supposed by M. Morlot to have required 10,000 years for its accumulation. At the