1. Hann: Handbuch der Klimatologie [Handbook of Climatology], Stuttgart, 1883, p. 1.
2. Lyell: Principles of Geology, 10th ed., Chap. X—XIII.
3. Lyell: Principles of Geology, 10th ed., Chap. X—XIII.
4. Heer: Flora Fossilis Helvetiae.
5. Lesquereux: A Review of the Fossil Flora of North America, Bull. of the Geol. and Geog. Survey of the Territories, II. Ser. no. V.