But after the formation of a crust and its covering in whole or in part with heated water, other changes must occur, in order to fit the earth for the abode of life. These proceeded from the tensions set up by the contraction and expansion of the interior heated nucleus and the solid crust--a complicated and difficult question, when we consider its laws and their mode of operation, but which resulted in the folding and fracturing of the crust along long lines which are parts of great circles of the earth, running in N.E. and S.W. and N.W. and S.E. directions; and these ridges, which in the earliest Archæan period must have attained to great height and very rugged outlines, formed the first rudiments of our mountain chains and continents. Those constituting the Laurentian nucleus of North America--a very simply outlined continent--form a case in point (Fig. 18). The elevation of these mountain ridges forced the waters to recede into the lower levels. As the old psalm of creation has it,-- "The mountains ascend, the valleys descend into the place Thou hast founded for them," and so sea-basins and land were produced. Milton merely paraphrases this when he says,-- "The mountains huge appear Emergent, and their broad, bare backs upheave Into the clouds; their tops ascend the sky. So high as heaved the tumid hills, so low Down sunk a hollow bottom wide and deep. Capacious bed of waters." Englishmen have been accused of taking their ideas of creation from Milton rather than from nature or the Bible. Milton had not the guidance of modern geology. His cosmology is entirely that of a close student of the Biblical narrative of creation. He is in many respects the best commentator on the early chapters of Genesis, because he had a very clear conception of the mind of the writer, and the power of expressing the ideas he derived from the old record. For the same reason he is the greatest bard of creation and primitive man, and surprisingly accurate and true to nature. [Illustration: Fig. 18.--_Map of Laurentian, North America._ Showing the protaxis or nucleus of the continent.] Then began the great processes of denudation and sedimentation to which we owe the succeeding rock formations. The rains descended on the mountain steeps, and washed the decaying rocks as sand, gravel and mud into the rivers and the sea. The sea itself raged against the coasts, and cut deeply into their softer parts; and all the detritus thus produced by atmospheric and marine denudation was spread out by the tides and currents in the bed of the ocean, and its gulfs and seas, forming the first aqueous deposits, while the original land must have been correspondingly reduced.