great in this part of the Alps that their original position is often inverted. Therefore the age of the granite is doubtful.

Plutonic rocks of Carboniferous Period.—The granite of Dartmoor, in Devonshire, was formerly supposed to be one of the most ancient of the plutonic rocks, but is now ascertained to be posterior in date to the Culm-measures of that county, which from their position, and as containing true coal-plants and Trilobites of the Phillipsia group, are now known to be members of the Carboniferous series. This granite has broken through the Devonian and Carboniferous stratified formations, the successive members of the Culm-measures abutting against the granite, and becoming metamorphosed as they approach it. These strata are also penetrated by granite veins, and dikes, called 'elvans." The granite of Cornwall is probably of the same date, and, therefore, as modern as the Carboniferous strata, if not newer.

Plutonic rocks of Silurian Period. It has long been thought that a very ancient granite near Christiania, in Norway, is posterior in date to the Lower Silurian strata of that region, although its exact position in the Paleozoic series cannot be defined. Von Buch first announced, in 1813, that it was of newer origin than certain limestones containing Orthocerata and Trilobites. The proofs consist in the penetration of granite veins into the shale and limestone, and the alteration of the strata, for a considerable distance from the point of contact, both of these Fig. 617.

Silurian.

Granite.

Silurian strata.

veins and the central mass from which they emanate. (See fig. 617.) When the junctions of the strata and the granite are carefully examined, it is found that the plutonic rock intrudes itself in veins and nowhere covers the fossiliferous strata in large overlying masses, as is so commonly the case with trappean formations.1

Now this granite, which is more modern than the Silurian strata of Norway, also sends veins into an ancient formation of gneiss of the same country; and the relations of the plutonic rock and the gneiss, at their junction, are full of interest when

9 Proceed. Geol. Soc. vol. ii. p. 562; and Trans. 2nd ser. vol. v. p. 686.

1 See the Gæa Norvegica and other works of Keilhau, with whom I examined this country.

we duly consider the wide difference of epoch which must have separated their origin.

The length of this interval of time is attested by the following facts:-The fossiliferous, or Silurian, beds rest unconformably upon the truncated edges of the gneiss, the inclined strata of which had been denuded before the sedimentary beds were superimposed (see fig. 618). The signs of denudation are two

Fig. 618.

Gneiss.

Granite.

Gneiss.

Granite sending veins into Silurian strata and gneiss, Christiania, Norway.

fold: first, the surface of the gneiss is seen occasionally, on the removal of the newer beds containing organic remains, to be worn and smoothed; secondly, pebbles of gneiss have been found in some of these Silurian strata. Between the origin, therefore, of the gneiss and the granite there intervened, first, the period when the strata of gneiss were denuded; secondly, the period of the deposition of the Silurian deposits on the denuded and inclined gneiss, a. The granite produced after this long interval is often so intimately blended with the gneiss at the point of junction, that all distinction is arbitrary.

Archæan Plutonic rocks.—Granite appears to have intruded upon the very metamorphic rocks which are the lowest in the South Wales area-the Dimetian of Dr. Hicks; and it is possible that the veins of it did not pass beyond this lowest horizon. The Laurentian rocks of Canada have important veins and dikes of diabase, sometimes of great thickness, and they are cut across by extensive masses of syenite, with veins on their contour, of reddish-brown felsite porphyry. These intrusive rocks appear not to enter the superimposed Silurians. But it is very evident that most of the eruptive rocks of the Archaic formations were subsequent, and occurred during the Devonian or Carboniferous age.

The intrusion of plutonic rocks into the gneisses and mica schists of Archæan and subsequent ages is exceedingly interesting, and especially when fragments of the schistose rocks are found included in the plutonic vein. Very frequently there is great difficulty in determining whether a rock is a true granite or is a gneiss, on account of some linear arrangement of the crystals, produced by pressure during consolidation.

CHAPTER XXXII.

METAMORPHIC ROCKS.

The terms Metamorphic and Metamorphism applied to rocks-Local or contact metamorphism-Regional metamorphism-All classes of rocks subject to metamorphism-Influence of heat, pressure, chemical action -The origin of the heat-Pressure-Anomalies-Direction of the pressure-Movement- Causes-Origin of oldest metamorphic rocksTheories of Von Lasaulx, Sterry Hunt, and others-The most metamorphosed rocks the oldest-Arguments for and against the theoryChemical changes-Dolomite, red chalk-Local or contact metamorphism, further examples regarding fossiliferous and carbonaceous rocks -Geikie's summary of local effects-Volcanic rocks metamorphosed— Metamorphic rocks altered - Regional metamorphic rocks — Gneiss, Mica-schists, Clay-slate, Rhyolite, Talc-schist, Hornblende-schist, Quartzite, Hälleflinta, Porcellanite, Crystalline limestone-Metamorphosed granite-Gabbro-Diabase-Serpentines.

We have now considered all the classes of rocks, except the last group, which comprises those called Metamorphic, and which are the result of the metamorphism of the others. The term Metamorphic implies that rocks have undergone changes of chemical, mineralogical, and textural kinds, and that their internal construction and outward appearance no longer resemble those of the original rock. Such changes and alterations as are sufficient to produce a kind of metamorphism, may be studied at the present day in volcanic regions, such as Iceland, or near Naples. The flowing of lava over soil, or into little streams or small lakes, produces remarkable alterations in the clays, which it bakes with heat and infiltrates with siliceous solutions, altering them chemically and mechanically. Similar changes occurred under the same circumstances in the geological ages, and may be taken as examples of local or contact metamorphism. But on examining the rocks in the midst of great mountain chains, and on the flanks of very old hills, schists, slates, crystalline limestones, gneisses, serpentines, &c., are found in positions where originally horizontal rocks have been subjected to vast lateral pressure, to superincumbent weight, to heat and the action of percolating gases, and chemical solutions. Such rocks have undergone regional metamorphism.'

Great extents of country have been thus altered. The mountains of North Wales, the Lake district, and the Alps exemplify the grand phenomena of metamorphism, much surpassed,

L L

however, in intensity in the North-west Highlands, where this regional metamorphism' is fully developed. There are then two groups of metamorphic rocks; in one the rocks have been locally affected, and in the other they have been exposed to more general and diffused agencies. Therefore geologists consider metamorphic action as local and as regional.

It is perfectly certain that metamorphism is not restricted to one class of rocks alone, but that the sedimentary strata, volcanic rocks, and even the plutonic, have often been altered, and are now presented to us, in many instances, under very different aspects. The degree of metamorphism varies—it may be intense or very slight; and the kind of change appears to have depended upon the original mineralogy of the rock and of its surrounding strata, together with the amount of pressure, heat, and introduced chemical matters.

The amount of heat available was great (as it is now) in the production of local, or contact, metamorphism; pressure was not necessarily great, and the intensity of the chemical action of the escaping steam and water charged with gases, and minerals in solution, was enhanced by certain amounts of it.

In the instances of regional metamorphism, the amount of heat which influenced the rocks appears to have been very variable in amount. The researches into the amounts of underground temperature at the present day lead us to believe that at a depth of little more than 10,000 feet 212° F. would be registered; and as the internal heat has been conducted to and radiated from the surface of the earth since the beginning, it follows that a higher temperature existed at this depth in the earlier geological ages.

During the great movements to which the strata of regions now occupied by mountain chains have been subjected, subsidences of 10,000 feet were minor phenomena, and similar and even much greater downward movements happened, during the progressive collection of many deep sedimentary strata (for example, of the Carboniferous formation), on areas where there has not been great crushing or the upheavals requisite for the development of a mountain system. The questions arise, If rocks have been sunken down to within the range of the temperature mentioned, or even of a higher, how have they been influenced and metamorphosed? Have they invariably been altered? If not, why not? It appears that some rocks have been considerably modified under the circumstances, but others have not. Amongst the examples of unaltered strata are the limestones, grits, and Coal measures, deposits which have been sunken down many thousands of feet, yet the alterations have

been but slight. Mallet urged with great ability, that heat over and above that due to the primitive state of the earth, was developed during the motion of rocks by the curving, crushing, and lateral movements incident to the formation of mountains and highly disturbed areas. This extra amount of heat was doubtless of practical value, and it is a matter of common observation that highly contorted deep strata are usually the most metamorphosed. But it is not invariably the case that such strata have been greatly altered; and it must be admitted (as Sterry Hunt has very well urged upon geologists) that very greatly curved, crushed, reversed, and dislocated strata on the flanks of the Alps have been but little or not at all metamorphosed. In fact, the difficulties of explaining the comparatively minor metamorphism of the later rocks which are on the flanks of mountain chains which are very old, and have been the result of several consecutive crust movements, are almost insuperable.

The amount and duration of the pressure, whether it was from side to side, or from above downwards also, were important factors, especially when the influence of percolating chemical solutions is admitted to have been very great. Pressure and heat enhance chemical action, and solutions were doubtless circulated amongst the rocks under the influence of a pressure which antagonised the great temperature, and did not permit the flashing off into steam. The slow removal of minerals, and their re-deposition, the decomposition of such complicated minerals as the felspars and micas, and the inevitable presence of carbon dioxide, all caused by and also associated with heat and long-continued pressure, give a slight insight into this process, the modern examples of which are wanting. It is necessary to remark that the pressure accompanying metamorphic action was both lateral and from above downwards, and possibly occasionally from below upwards, in direction. Gneiss, which is a foliated rock, usually has its layers parallel with the plane of the original bedding; but most of the schistose rocks have been subjected to vast, intermittent, long lasting, lateral thrust, accompanied by pushing, over-rolling movements. Their original bedding is no longer visible; but cleavage planes have been developed perpendicular to the direction of the force, and also nearly to that of the original bedding planes. The fossils which were present before the movement began, are often found crushed and also deformed, squeezed obliquely, and as if they had been rolled, partially, on their long axes. The causes of the lateral pressure have been considered (p. 52), and it is very probable that it was more intense before secular cooling had persisted for a great time.