Part 8

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. } }

3. Newer Pliocene. } Pliocene. } 4. Older Pliocene. } } } 5. Upper Miocene. } Miocene. } Tertiary } 6. Lower Miocene. } } or } } Cainozoic. } 7. Upper Eocene. } } } 8. Middle Eocene. } 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 or } 31. Middle } Devonian. { Old Red } } 32. Lower } { Sandstone. } Primary } } or } Palæozoic. 33. Upper } } Palæozoic. } 34. Lower } Silurian. Silurian. } } 35. Upper } } 36. Lower } Cambrian. Cambrian. } } 37. Upper } } 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:--

{Cambrian, Silurian, and Devonian 73 Palæozoic {Carboniferous 683 {Permian 97

{Triassic 115 Mesozoic {Jurassic 294 {Cretaceous 183

{Eocene 414 Cainozoic {Miocene 496 {Pliocene 35

Recent Post-Pliocene 31 ---- Fossil Species. 2421

During the twenty years that have elapsed since this enumeration was made, the number of fossil species has been very greatly increased. The proportion exhibited in this table is likewise greatly altered from the enormous additions made to the Tertiary Flora by Unger, Ettingshausen, and Heer, and from the important contributions by Principal Dawson to the Devonian Flora.

Among the fossil Thalamifloral Dicotyledons, Unger mentions species belonging to the orders--

Magnoliaceæ. Anonaceæ. Nymphæaceæ. Capparidaceæ. Malvaceæ. Byttneriaceæ. Tiliaceæ. Aurantiaceæ. Malpighiaceæ. Aceraceæ. Sapindaceæ. Cedrelaceæ. Zygophyllaceæ. Xanthoxylaceæ. Coriariaceæ.

Among Calycifloral Dicotyledons--

Celastraceæ. Rhamnaceæ. Anacardiaceæ. Amyridaceæ. Leguminosæ. Rosaceæ. Calycanthaceæ. Combretaceæ. Melastomaceæ. Myrtaceæ. Halorageaceæ. Cucurbitaceæ. Cornaceæ. Loranthaceæ. Rubiaceæ.

Among Corollifloral Dicotyledons--

Ericaceæ. Styracaceæ. Ebenaceæ. Aquifoliaceæ. Sapotaceæ. Oleaceæ. Apocynaceæ. Gentianaceæ.

Among Monochlamydeous Angiosperms--

Nyctaginaceæ. Lauraceæ. Proteaceæ. Aquilariaceæ. Samydaceæ. Santalaceæ. Euphorbiaceæ. Urticaceæ. Artocarpaceæ. Ceratophyllaceæ. Salicaceæ. Myricaceæ. Betulaceæ. Altingiaceæ. Platanaceæ. Corylaceæ. Juglandaceæ. Rafflesiaceæ.

Among Monochlamydeous Gymnosperms--