As the surface cooled, a crust formed, and lay like a blanket over the liquid mass beneath. Convection currents—large-scale boiling movements—stirred the molten interior, thrust portions of the crust upward, and sucked other portions downward to be remelted. Some of the lighter components, such as compounds of silicon and oxygen and hydrogen, accumulated on the surface like froth on a kettle: the continents were born. With further cooling the atmosphere and oceans came into being. Something can be told of the age of the continents. Measurements of radioactivity in the most ancient rocks exposed at the surface today indicate that the oldest known continental rock is between three and four billion years old. Since the continents were formed, they have been bent and shifted and broken by the pressures exerted against them by convection in the interior. Parts of the continents at times have been submerged below the level of the sea, even as they are today. Other portions, lifted above sea level, were immediately attacked by the wearing-down processes of erosion. The battle between mountain-building forces and erosion has been a continuous one ever since the crust was formed. Even now earthquakes give testimony to continued crustal movement, storms still sweep across the continents and wash mud and frost-loosened rocks into churning torrents, rivers still deposit great floodplains and deltas, sediments accumulate slowly but persistently upon the bottoms of the seas. PRECAMBRIAN ERA Only part of the earth’s very early history is represented in Colorado, where the oldest known rocks are the gneisses and schists of the Idaho Springs Formation, at least 1,800,000,000 years old. These rocks appear to be the remains of ancient sediments, folded and metamorphosed into vast mountain areas long before recognizable life inhabited the earth. Precambrian rocks in Colorado are on the whole very poorly known. They have, however, been studied in detail in the Front Range west of Denver and Boulder, where they have been intensively explored for valuable minerals. The lack of fossils in the oldest rocks makes their close correlation difficult, but from studies of radioactive minerals contained in these rocks, and of the relationships of the rock units themselves, we can list them in order of their relative ages. Note that the rock sequence given below reads chronologically from bottom to top—a logical pattern in geology since younger rocks, especially those of sedimentary origin, normally lie above older ones. Recent studies indicate that the sequence may be much more complex than shown here.