granite that there is a passage between the two kinds of rocks; and that where the slaty shales impinge upon the granite, the line of junction is easily distinguishable. They are, in fact, cases of truly intrusive granite surrounded by aureoles of metamorphic rocks like those already described; the felspathic sandstones being more susceptible of metamorphism than the shales, and containing moreover the precise ingredients for transformation into a granitoid rock.

The cases in which layers of granite or granitoid rock are distinctly interbedded with thick beds of mica-schist and gneiss are at first sight more convincing, because when traced in one direction the beds of granite and gneiss become thicker, and the intervening bands of mica-schist are thinner and fewer, till at last the ground is wholly occupied by granite. Such a case is to be found in the district of Errigal, county Donegal, Ireland; but this may, perhaps, be explained by supposing the whole metamorphic area to be underlain by a mass of granite comparable to that of Leinster. The diverse strata dipping down on to this granite have been more or less metamorphosed according to their mineralogical composition,-pure sandstone into quartzite, limestone into marble, shales into mica-schist, and certain bands of mud-stones or felspathic sandstones into gneiss and granitoid rock.'

Notwithstanding the doubt that hangs over the metamorphic origin of certain granite, the possibility has been very generally accepted; and just as a connection has been traced between some granites and certain volcanic rocks, so it has been thought that other granites may result from the extreme stage of metamorphism, and that the process which can produce gneiss only requires to be carried a stage further for the production of granite.

On this subject Dr. A. Geikie thus expresses himself: 2 "It seems to me not only conceivable, but probable, that, whether melted by depression into the heated interior of the earth, or by the high temperature produced during

Originally described by R. H. Scott, "Journ. Roy. Geol. Soc. of Ireland," vol. i. p. 144; vide account in Green's "Physical Geology," second edition, p. 265.

2 In "Trans. Edin. Geol. Soc." vol. ii. pt. iii.

compression and contortion, granite may sometimes have been formed in the midst of crumpled and metamorphosed rocks, and intruded into them in bosses and veins; while, at other times and places, coming in contact with the aqueous circulation in the upper part of the crust, it gave rise to the ordinary phenomena of volcanoes. Hence I can believe that, even in the same mountain range, one mass of granite may be simply the extreme form of the metamorphism of rocks, in situ, under which it remained always deeply buried until exposed by denudation, while another mass, originally formed in exactly the same way, may be the lower end of a column of melted material of which the upper portion reached the surface of the earth, producing there streams of lava and showers of ashes."

We may, therefore, with Sir Charles Lyell, regard the whole crust of our globe as consisting of materials passing through an endless cycle of mutations, existing at one time as igneous rocks, then gradually decomposed, broken up, sorted and deposited as aqueous rocks, buried under fresh accumulations, metamorphosed and ultimately reabsorbed into the igneous rocks. In this view the most highly metamorphosed rocks would be those hovering upon the brink of re-absorption, and gneiss accordingly on the brink of passing into granite, and often almost undistinguishable from it.

THE

PART III.

PHYSIOGRAPHICAL GEOLOGY.

'HE first chapter of this book treated of the earth as a whole, and the opinions entertained with regard to the general structure of its mass and the condition of its interior were there given. In the chapters of Part II. the structure and arrangement of the rock-masses which compose the superficial portion of the earth's crust were described. We are now in a position to deal with the problems of Physiographical Geology, or that branch of the science which seeks to account for the existence of the varied physical features of the earth's surface, and explains the manner in which oceans and continents, mountains and plains, hills and valleys, have been gradually developed out of the rock-masses by the machinery of the physical agencies

described in Part I.