Followed a long period of milder climate indicated in America by thick deposits of limestone with the remains of reef-building organisms and other marine life. This period may have been interrupted at least once by the recurrence of glacial conditions, but the evidence for this is doubtful. It must be remembered that the duration of the Proterozoic was very great, at least as long as all subsequent time, while the relics of it which are now known to us are few and scattered, so that much which happened during that time is a closed book. It is not until the very close of the Proterozoic that we again find indisputable evidence of widespread glacial action. This second great glaciation was placed originally in the earliest Cambrian (see table of geological formations at the end of Chapter I), but later evidence shows that it is slightly older than the oldest deposit which can be referred to this series, and it may be designated the Pre-Cambrian glaciation. Tillites of this age have been found in the middle Yangtse region of China and in South Australia (where they extend from 20 miles south of Adelaide to 440 miles north, with an east-west extension of 200 miles). Glacial deposits which probably refer to this period have been found also in India, both in the Deccan and near Simla, over a wide area in South Africa, and in the extreme north of Norway. This distribution suggests the presence of two centres of glaciation, one between China, India and Australia, and the other north-west of Europe. The south-eastern of these was the most extensive, and probably indicates a ring of glaciated continents surrounding the pole, rather than a single enormous ice-sheet. During the Cambrian all evidence of glacial action ceases, and we have, instead, evidence of a warm, fairly uniform climate in the abundant marine life. This continued during the Ordovician and became accentuated during the Silurian period, when reef corals lived in the seas of all parts of the world. Terrestrial deposits are curiously lacking in all this series, and this suggests that in the absence of any great mountain-building and elevation shallow seas extended over almost the whole of the surface, accompanied by mild oceanic climates extending to high latitudes.