A Manual of Elementary Geology Part 56

23. Middle Trias { Compact greyish limestone } With Equiset.i.tes or { with beds of dolomite and } and Calamite.

Muschelkalk. { gypsum,--North of Germany, } { p. 287. Wanting in } { England. }

24. Lower Trias. { Variegated or Bunter sandstone } Plants different for { of Germans--Red and white } the most part from { spotted sandstone with } those of the Upper { gypsum and rock-salt, P. 288 } Trias.

{ } { Part of New Red sandstone of } { of Ches.h.i.+re with rock-salt, } { p. 294. }

IV. PRIMARY.

K. PERMIAN.

25. Upper Permian. { Yellow magnesian limestone, } Organic remains, both { Yorks.h.i.+re and Durham, } animal and vegetable, { P. 301. } more allied to primary { } than to secondary { Zechstein of Thuringia, Upper } periods.

{ part of Permian beds, } { Russia. }

26. Lower Permian. { _a._ Marl slate of Durham and } Thecodont saurians.

{ Thuringia. } Heterocercal fish of { } genus Palaeoniscus, &c.

{ _b._ Lower New Red sandstone } { of north of England and } { Rothliegendes of Germany. } { } { _a._ and _b._ Lower part of } { Permian beds, Russia, } { p. 301. }

L. CARBONIFEROUS.

27. Coal measures. { _a._ Strata of sandstone and } Great thickness of { shale, with beds of } strata of { coal,--S. Wales and } fluvio-marine origin, { Northumberland, p. 309. } with beds of coal of { } vegetable origin, { _b._ Millstone grit,--S. } based on soils { Wales, Bristol coal-field, } retaining the roots { Yorks.h.i.+re, p. 308. } of trees.

} } Oldest of known reptiles } or Archegosaurus.

} Sauroid fish.

28. Mountain { Carboniferous or mountain { Brachiopoda of genus limestone. { limestone, with marine { Productus.

{ sh.e.l.ls and corals. { { { Cephalopoda of genera { Mendip Hills, and many parts { Cyrtoceras, Goniat.i.te, { of Ireland, p. 340. { Orthoceras.

{ { Crustaceans of the { genus Phillipsia.

{ { Crinoideans abundant.

M. DEVONIAN.

29. Upper { _a._ Yellow sandstone of Dura } Tribe of fish with hard Devonian. { Den, Fife. } coverings like { } chelonians, { _b._ Red sandstone and marl } Pterichthys, { with cornstone of } Pamphractus, &c.; { Herefords.h.i.+re and } also of genera { Forfars.h.i.+re. } Cephalaspis, { } Holoptichius, &c.

{ Paving and roofing-stone, } { Forfars.h.i.+re. } No reptiles yet known.

{ } { Upper part of Devonian beds } { of South Devon. }

30. Lower { Grey sandstone with } Fish, partly of same Devonian. { Ichthyolites,--Caithness, } genera, but of { Cromarty, and Orkney, Lower } distinct species from { part of Devonian beds of } those in Upper { South Devon, and green } Devonian; also { chloritic slates of } Osteolepis, { Cornwall, limestone of } Coccosteus, { Gerolstein, Eifel. } Glyptolepis, } Dipterus, &c.

N. SILURIAN.

31. Upper { _a._ Tilestone of Brecon and { Oldest of fossil fish Silurian. { Caermarthen. { yet discovered.

{ { { _b._ Limestone and shale, { Trilobites and { Ludlow, Shrops.h.i.+re. { Graptolites abundant.

{ { { _c._ Wenlock or Dudley { Brachiopoda very { limestone. { numerous.

{ { Cephalopoda: { Bellerophon, { Orthoceras.

{ Same genera of 32. Lower { _a._ Caradoc sandstone, Caer { invertebrate animals Silurian. { Caradoc, Shrops.h.i.+re. { as in Upper Silurian, { { but species chiefly { _b._ Llandeilo flags, { distinct. Trinucleus { calcareous flags and { caractaci, Cystideae, { schists,--Builth, { p. 358.

{ Radnors.h.i.+re, Llandeilo, { { Caermarthens.h.i.+re. { No land plants yet { known.

{ { Footprints of tortoise, { see note, p. 360.

FOOTNOTES:

[352-A] Murchison, Silurian System, p. 198, 199.

[354-A] Silurian System, pl. 7. bis. fig. 1. b.

[358-A] Quart. Geol. Journ., vol. ii. p. 11.; and Memoirs of Geol. Survey, vol. ii. p. 518.

[359-A] Quart. Geol. Journ., vol. iv. p. 300.

[359-B] Ibid., 299.

[359-C] Ibid., 145.

[360-A] Since this was written, Mr. Logan has discovered chelonian footprints in the lowest fossiliferous beds of the Silurian series, near Montreal, in Canada. Professor Owen inclines to refer them to the genus _Emys_.--_Quart. Journ. G. S._, vol. vii. p. lxxvi.

CHAPTER XXVIII.

VOLCANIC ROCKS.

Trap rocks--Name, whence derived--Their igneous origin at first doubted--Their general appearance and character--Volcanic cones and craters, how formed--Mineral composition and texture of volcanic rocks--Varieties of felspar--Hornblende and augite--Isomorphism--Rocks, how to be studied--Basalt, greenstone, trachyte, porphyry, scoria, amygdaloid, lava, tuff--Alphabetical list, and explanation of names and synonyms, of volcanic rocks--Table of the a.n.a.lyses of minerals most abundant in the volcanic and hypogene rocks.

The aqueous or fossiliferous rocks having now been described, we have next to examine those which may be called volcanic, in the most extended sense of that term. Suppose _a a_ in the annexed diagram, to represent the crystalline formations, such as the granitic and metamorphic; _b b_ the fossiliferous strata; and _c c_ the volcanic rocks. These last are sometimes found, as was explained in the first chapter, breaking through _a_ and _b_, sometimes overlying both, and occasionally alternating with the strata _b b_. They also are seen, in some instances, to pa.s.s insensibly into the unstratified division of _a_, or the Plutonic rocks.

[Ill.u.s.tration: Fig. 434. Cross section.

_a._ Hypogene formations, stratified and unstratified.

_b._ Aqueous formations.

_c._ Volcanic rocks.]

When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanos. They also found certain rocks, for the most part without stratification, and of a peculiar mineral composition, to which they gave different names, such as basalt, greenstone, porphyry, and amygdaloid. All these, which were recognized as belonging to one family, were called "trap" by Bergmann, from _trappa_, Swedish for a flight of steps--a name since adopted very generally into the nomenclature of the science; for it was observed that many rocks of this cla.s.s occurred in great tabular ma.s.ses of unequal extent, so as to form a succession of terraces or steps on the sides of hills. This configuration appears to be derived from two causes. First, the abrupt original terminations of sheets of melted matter, which have spread, whether on the land or bottom of the sea, over a level surface. For we know, in the case of lava flowing from a volcano, that a stream, when it has ceased to flow, and grown solid, very commonly ends in a steep slope, as at _a_, fig. 435. But, secondly, the step-like appearance arises more frequently from the mode in which horizontal ma.s.ses of igneous rock, such as _b c_, intercalated between aqueous strata, have, subsequently to their origin, been exposed, at different heights, by denudation. Such an outline, it is true, is not peculiar to trap rocks; great beds of limestone, and other hard kinds of stone, often presenting similar terraces and precipices: but these are usually on a smaller scale, or less numerous, than the volcanic _steps_, or form less decided features in the landscape, as being less distinct in structure and composition from the a.s.sociated rocks.

[Ill.u.s.tration: Fig. 435. Step-like appearance of trap.]

Although the characters of trap rocks are greatly diversified, the beginner will easily learn to distinguish them as a cla.s.s from the aqueous formations. Sometimes they present themselves, as already stated, in tabular ma.s.ses, which are not divided into strata: sometimes in shapeless lumps and irregular cones, forming chains of small hills.

Often they are seen in dikes and wall-like ma.s.ses, intersecting fossiliferous beds. The rock is occasionally found divided into columns, often decomposing into b.a.l.l.s of various sizes, from a few inches to several feet in diameter. The decomposing surface very commonly a.s.sumes a coating of a rusty iron colour, from the oxidation of ferruginous matter, so abundant in the traps in which augite or hornblende occur; or, in the felspathic varieties of trap, it acquires a white opaque coating, from the bleaching of the mineral called felspar. On examining any of these volcanic rocks, where they have not suffered disintegration, we rarely fail to detect a crystalline arrangement in one or more of the component minerals. Sometimes the texture of the ma.s.s is cellular or porous, or we perceive that it has once been full of pores and cells, which have afterwards become filled with carbonate of lime, or other infiltrated mineral.

Most of the volcanic rocks produce a fertile soil by their disintegration.

It seems that their component ingredients, silica, alumina, lime, potash, iron, and the rest, are in proportions well fitted for vegetation. As they do not effervesce with acids, a deficiency of calcareous matter might at first be suspected; but although _the carbonate_ of lime is rare, except in the nodules of amygdaloids, yet it will be seen that lime sometimes enters largely into the composition of augite and hornblende. (See Table, p. 377.)

_Cones and Craters._--In regions where the eruption of volcanic matter has taken place in the open air, and where the surface has never since been subjected to great aqueous denudation, cones and craters const.i.tute the most striking peculiarity of this cla.s.s of formations. Many hundreds of these cones are seen in central France, in the ancient provinces of Auvergne, Velay, and Vivarais, where they observe, for the most part, a linear arrangement, and form chains of hills. Although none of the eruptions have happened within the historical era, the streams of lava may still be traced distinctly descending from many of the craters, and following the lowest levels of the existing valleys. The origin of the cone and crater-shaped hill is well understood, the growth of many having been watched during volcanic eruptions. A chasm or fissure first opens in the earth, from which great volumes of steam and other gases are evolved. The explosions are so violent as to hurl up into the air fragments of broken stone, parts of which are s.h.i.+vered into minute atoms. At the same time melted stone or _lava_ usually ascends through the chimney or vent by which the gases make their escape. Although extremely heavy, this lava is forced up by the expansive power of entangled gaseous fluids, chiefly steam or aqueous vapour, exactly in the same manner as water is made to boil over the edge of a vessel when steam has been generated at the bottom by heat. Large quant.i.ties of the lava are also shot up into the air, where it separates into fragments, and acquires a spongy texture by the sudden enlargement of the included gases, and thus forms _scoriae_, other portions being reduced to an impalpable powder or dust. The showering down of the various ejected materials round the orifice of eruption gives rise to a conical mound, in which the successive envelopes of sand and scoriae form layers, dipping on all sides from a central axis. In the mean time a hollow, called a _crater_, has been kept open in the middle of the mound by the continued pa.s.sage upwards of steam and other gaseous fluids. The lava sometimes flows over the edge of the crater, and thus thickens and strengthens the sides of the cone; but sometimes it breaks it down on one side, and often it flows out from a fissure at the base of the hill (see fig. 436.).[368-A]

[Ill.u.s.tration: Fig. 436. Part of the chain of extinct volcanos called the Monts Dome, Auvergne. (Scrope.)]

_Composition and nomenclature._--Before speaking of the connection between the products of modern volcanos and the rocks usually styled trappean, and before describing the external forms of both, and the manner and position in which they occur in the earth's crust, it will be desirable to treat of their mineral composition and names. The varieties most frequently spoken of are basalt, greenstone, syenitic greenstone, clinkstone, claystone, and trachyte; while those founded chiefly on peculiarities of texture, are porphyry, amygdaloid, lava, tuff, scoriae, and pumice. It may be stated generally, that all these are mainly composed of two minerals, or families of simple minerals, _felspar_ and _hornblende_; some almost entirely of hornblende, others of felspar.

These two minerals may be regarded as two groups, rather than species.