Introduction to the Study of Palæontological Botany - John Hutton Balfour

Before drawing conclusions as to the climate or physical condition of the globe at different geological epochs, the botanist must be well informed as to the vegetation of different countries, as to the soils and localities in which certain plants grow, whether on land or in the sea, or in lakes, in dry or marshy ground, in valleys or on mountains, or in estuaries, in hot, temperate, or cold regions. Great caution must be employed also in predicating from one species the conditions of another, inasmuch as different species of the same genus frequently exist in very different habitats, and under almost opposite conditions of moisture and temperature. It is only by a careful consideration of all these particulars that any probable inferences can be drawn as to the condition of the globe. Considering the physiognomy of vegetation at the present day, we find remarkable associations of forms. The Palms, although generally characteristic of very warm countries, are by no means confined to them; Chamærops humilis extending to Europe as far as lat. 43° to 44° N., and C. palmetto in North America to lat. 34° to 36° N., while C. Fortunei, from the north of China, is perfectly hardy in the south of England. Major Madden mentions the association of Palms and Bamboos with Conifers at considerable elevations on the Himalayas. (Edin. Bot. Soc. Trans. iv., p. 185.) Epiphytic Orchids, which usually characterise warm climates, have representatives at great elevations, as Oncidium nubigenum at 14,000 feet in the Andes, and Epidendrum frigidum at from 12,000 to 13,000 feet in the Columbia mountains. These facts point out the care necessary before drawing conclusions as to the climate which fossil plants may be supposed to indicate.

[Fossiliferous Rocks.]

The rocks of which the globe is composed are divided into two great classes—the Stratified or Aqueous, and the Unstratified or Igneous. The stratified rocks frequently contain fossil remains, and are then called fossiliferous; those with no such remains are designated non-fossiliferous or azoic. The igneous unstratified rocks, included under the names of Granitic and Trappean, show no appearance of animal or vegetable remains. Those trap rocks, however, which have been formed of loose volcanic ashes have often enclosed and preserved the remains of plants and animals; while even between the successive beds of old lava-like trap rocks organic remains are sometimes found. Thus, in Antrim, near the Giant's Causeway, deposits containing vegetable remains occur inter-stratified with basaltic rocks. These remains are of Miocene age, and have been referred to coniferous plants, beeches, oaks, plane trees, etc. Similar plants have been discovered in a similar position by the Duke of Argyll in the island of Mull. In trap rocks near Edinburgh, lignite with distinct structure has also been detected. Silicified wood and coal, imbedded in trap rocks, have been seen in Kerguelen's Land. The wood is found enclosed in basalt, whilst the coal crops out in ravines, in close contact with the overlying porphyritic and amygdaloidal greenstone. Hooker has also seen silicified wood, in connection with trap, in Macquarrie's Plains, in Tasmania. Several beds of trap-tuff or ash, formed into solid compact rock by infiltrated carbonate of lime, occur in the north-east of Arran, which contain numerous stems, branches, and fruits of carboniferous plants. These represent the remains of successive forests which grew on this locality, and were one after the other destroyed by the ash-showers poured forth from a neighbouring volcano during its intermittent periods of activity.

Fossil remains are extremely rare in certain rocks, which, from the changes they have undergone, have been denominated Metamorphic. These include Gneiss and Mica-slate, which are stratified rocks subsequently altered by heat and other causes, and so completely metamorphosed that the traces of organisms have been nearly obliterated. Nevertheless, recognisable traces of plant and animal remains have been found in what were recently thought to be azoic rocks. The absence of organic remains in rocks is therefore not sufficient to enable us to state that these rocks were formed before animals or vegetables existed.

The stratified rocks which contain fossils have been divided into three great groups—the Palæozoic, the Secondary, and the Tertiary, or into Palæozoic and Neozoic groups. The formations included under these are exhibited in the following table, taken from Lyell's Manual of Geology:—

1.	Recent.	}	Post Tertiary.	}	Recent.
2.	Post Pliocene.	}	Post Tertiary.	}	Recent.

3.	Newer Pliocene.	}	Pliocene.	}	Tertiary or Cainozoic.	}
4.	Older Pliocene.	}	Pliocene.	}		}
				}		}
5.	Upper Miocene.	}	Miocene.	}		}
6.	Lower Miocene.	}	Miocene.	}		}
				}		}
7.	Upper Eocene.	}	Eocene.	}		}
8.	Middle Eocene.	}		}		}
9.	Lower Eocene.	}		}		}	Neozoic.
						}
10.	Maestricht Beds.	}		}		}
11.	White Chalk.	}		}		}
12	Chloritic Series.	}		}	Secondary	}
13.	Gault	}	Cretaceous.	}	or	}
14.	Neocomian.	}		}	Mesozoic.	}
15.	Wealden.	}		}		}

16.	Purbeck Beds.	}		}		}
17.	Portland Stone.	}		}		}
18.	Kimmeridge Clay.	}		}		}
19.	Coral Rag.	}	Jurassic.	}		}
20.	Oxford Clay.	}		}	Secondary	}
21.	Great or Bath Oolite.	}		}	or	}	Neozoic.
22.	Inferior Oolite.	}		}	Mesozoic.	}
23.	Lias.	}		}		}
				}		}
24.	Upper Trias.	}		}		}
25.	Middle Trias.	}	Triassic.	}		}
26.	Lower Trias.	}		}		}

27.	Permian.		Permian.	}
				}
28.	Coal Measures.	}		}
29.	Carboniferous	}	Carboniferous.	}
	limestone.	}		}
				}
30.	Upper Devonian.	}	Devonian	}
31.	Middle Devonian.	}	or Old Red	}
32.	Lower Devonian.	}	Sandstone.	}	Primary	}
				}		}
33.	Upper Silurian.	}	Silurian.	}	or	}	Palæozoic.
34.	Lower Silurian.	}	Silurian.	}		}
				}	Palæozoic.	}
35.	Upper Cambrian.	}	Cambrian.	}
36.	Lower Cambrian.	}	Cambrian.	}
				}
37.	Upper Laurentian.	}	Laurentian.	}
38.	Lower Laurentian.	}	Laurentian.	}

[Natural Orders to which Fossil Plants belong.]

The plants found in different strata are either terrestrial or aquatic, and the latter exhibit species allied to the salt and fresh water vegetables of the present day. Their state of preservation depends much on their structure. Cellular plants have probably in a great measure been destroyed, and hence their rarity; while those having a woody structure have been preserved. The following is the number of fossil genera and species, as compiled from Unger's work on Palæophytology—(Unger, Genera et Species Plantarum Fossilium, 1850).

Dicotyledones.		Genera.	Species.
	Thalamifloræ.	24	84
	Calycifloræ	56	182
	Corollifloræ	23	60
	Monochlamydeæ Angiospermæ	48	221
	—————— Gymnospermæ	56	363
Monocotyledones.
	Petaloideæ	38	130
	Glumiferæ	5	12
Acotyledones.
	Thallogenæ	31	203
	Acrogenæ	121	969
	Doubtful	35	197
		——	——
		437	2421

These plants are arranged in the different strata as follows:—