nier, near Chambéry, on 23 December 1248; Uzielli however is forced to admit that the “ruina” recalled by Leonardo happened as a result of the succession of earthquakes which occurred in many areas of northern Italy in the years 1504-1506, and therefore the period around which the considerations below lead us to suppose that the Codex might have been written. It would appear to me that these pages, or at least most of them, were written during one of Leonardo’s stays in Florence, excepting the period of his youth. The examples which illustrate Vinci’s theories draw frequently from hydrographic and stratigraphic conditions in Tuscany and, although they are found mixed up with others referring to northern Italy, they may generally be distinguished on account of the precise and up to date observations lacking in the latter; and this appears to be true even when in the Codex there are not just observations but figures copied from other notebooks of the genius. The discovery of a prehistoric ship on the property of Messer Gualtieri, near Candia Lomellina; the phenomena which the formation of an immense cloud gave rise to, which Leonardo could see “già sopra a Milano, inverso lago Maggiore”(already above Milan, in the opposite direction to Lake Maggiore; the trip to Monte Rosa, which provided Leonardo with a demonstration of the cause of the blue in the sky are but vivid memories of the past. The Po Valley, the Lombard lakes and rivers, the Martesana Canal and the Grand Canal are mentioned without minute detail in the course of the considerations contained in this manuscript. Nor are there many notes on the fossils of Lombardy or on the parts of the Po Valley occupied by the sea in ancient times. But when Leonardo speaks of Tuscany, in a bold and enlightened explanation of his geological theory, he gives a detailed description of the changes taking place in the Arno valley with accurate indications of the places and fresh impressions of that area familiar to him containing the villages of Vinci,