The close of the early Carboniferous period was marked by an augmentation of the orogenic movements.
In intimate relationship with the mountain-building orogenic crustal movements was the prevalence of volcanic activity during the earlier part of this period.
The net result of the orogenic movements was, that at the close of the period there existed a great northern continental mass, embracing Europe, North Asia and North America; and a great southern continental mass, including South America, Africa, Australia and India.
The mountain structures originated in three great orogenic periods, the earliest in the Archean, the second at the end of the Palaeozoic and the third at the end of the Mesozoic. The Archean mountain chains, which enclosed the present region of Hudson Bay, were so ancient that they had already been worn down almost to a plain before the early Palaeozoic sediments were laid down.