Complex sentence: His sea maps encompassed not only Europe, Great Britain and Ireland but also the North Sea and the Baltic Sea; the Sea-Atlas covered the English Channel, the Mediterranean and the Arctic Ocean as also the Indian and Pacific Oceans.
Split into simpler sentences: His sea maps encompassed not only Europe, Great Britain and Ireland but also the North Sea and the Baltic Sea. The Sea-Atlas covered the English Channel, the Mediterranean and the Arctic Ocean as well as the Indian and Pacific Oceans.