many injections of granite have only penetrated the lower rocks without proceeding further. There is also a still more cogent reason why granite should occupy this position, viz., that all granite now found at the surface must be there in consequence of vast denudation having taken place, great masses of other rocks having been removed by detrition, together, perhaps, with much of the granite that once existed above the present surface. This denudation of course exposes the lower rocks to view, while parts of the higher rocks that were perhaps equally penetrated by the granite have been removed, the other parts which remain being at a distance from the granite, and showing no signs of such penetration. The relations of a large mass of granite to the adjoining rocks are generally rather complicated, the boundary line being very irregular, and the granite running out into tongues and bosses as already mentioned. Where the junction of granite with other rocks can be studied over a large area it suggests the arrangement exhibited in the diagram, fig. 161. The molten rock would seem to have forced its way upwards and sideways, eating away support after support of the mass above it, and probably in some cases actually melting them down and absorbing their materials into itself. The pressure from below has caused not only injections of the yet molten rock into the cracks and fissures of the superincumbent mass, but undulations in the general surface of the granite, some parts of the overlying mass being heaved up, and others sinking down into the granite. The upper surface of the granitic mass, on its final consolidation, would thus be an excessively irregular one, with protuberant mounds and deep hollows, while the beds of the superincumbent mass would not be likely to conform at all to this irregular surface, but would often dip directly down on to it, or abut against it in all kinds of ways, and with any amount of inclination. Fig. 161 may be taken as a diagrammatic illustration of a tract of stratified rock invaded by granite, G representing a mass of granite gradually forced upward into the mass of slate which had already been disturbed and tilted in various directions. The horizontal lines, ab, cd, ef, gh, indicate successive surfaces of denudation. So long as the a с surface was represented by ab, no part of the granite would be seen, and little metamorphism would be observable. If the denudation reached to the surface c d, some veins of granite would be visible, and these would become more frequent along the surface ef; intermediate surfaces between ef and gh would expose thick bosses of granite with many patches of micaceous schist upon them, and apparently dipping down into them; the width of the granite area would be wide in proportion to the depth of the denudation, and when a large area was exposed, the veins would only be visible round its borders. These ideas respecting the form of the original subterranean surface of granite masses are corroborated by an examination of the districts where large areas of granite are exposed. The largest surface exposure of granite in the b d f h Fig. 161. Diagram to illustrate the intrusion of Granite (after Jukes). British Islands is that of Leinster in Ireland; this granite area occupies large parts of the counties Wicklow, Carlow, and Wexford, being 70 miles long and from 7 to 17 miles wide. The rocks surrounding this area belong to two distinct formations, known respectively as the Cambrian and Ordovician systems, the former being the lower and older. Each of these are several thousand feet thick, and their component beds are greatly bent and contorted. Now although we are compelled to suppose that the granite must have come through the Cambrian rocks before it could penetrate the Ordovician, yet it is remarkable that in no instance does any part of the lower or Cambrian for mation come in contact with the granite at the surface, though it does come in some places within two or three miles of it. 482 STRUCTURAL GEOLOGY. This, therefore, is a sufficient refutation of the old idea that the intrusion of granite was the cause of elevation, and that the rocks in contact with it were always the lowest and oldest to be found in the country. In the Leinster district it is Silurian or newer rocks which are seen to be altered into gneiss and mica-schist by contact with the granite. "Moreover, in those parts where the granite forms lofty hills, the mica-schist spreads far up on the flank of those hills, and on the very loftiest, such as Lugnaquilla (which is over 3,000 feet above the sea), large patches of mica-schist occur even on the summit, so that the surface exposure of granite is there narrowest and most interrupted." Had the hills been left another 500 or 1,000 feet higher, the granite would apparently have been entirely concealed there by masses of mica-schist stretching completely over it. "On the other hand, where the granite forms low ground, as above Tullow and Hacketstown, its surface exposure is there by far the widest, and all the central part of it is completely free from patches of mica-schist. It is obvious that these differences are the result of the different amount of denudation that has acted on the granite. Where the ground is loftiest we have the nearest approach to the original surface of the granite and its original covering of other rock; where the denudation has cut down deepest, so as to form low ground, there we get deeper into the granite mass, or further from its original surface, to a depth, indeed, to which no mass of mica-schist could extend, unless it were altogether detached from the overlying mass, and enclosed in the granite."' It is therefore almost certain that the whole of Wicklow and Wexford is underlain by a continuous mass of granite The granite district of with a very irregular surface. the Mourne Mountains in county Down presents similar features.2 1 Quoted from Jukes' "Manual of Geology," second edition, p. 318. It should however be stated that the altered rocks cannot be properly called gneiss and mica-schist, though they may be described as micaceous schists. 2 See "Memoirs of the Geol. Survey of Ireland," Sheets 60, 61, 71, 72, one vol., 1881, and Hor. Section 22. CHAPTER XI. IGNEOUS ROCKS REGARDED AS ROCK-MASSES. B. VEINS, DYKES, SILLS, AND LACCOLITES. CONN ONNECTION of Plutonic and Volcanic Rocks. -From the descriptions given in the preceding chapter the reader will be prepared to accept the remark made long ago by Jukes, that "if we could follow any actual lava-stream to its source within the bowels of the earth, we should in all probability be able to mark in its course every gradation, from pumice or obsidian to actual granite." The probability of this might be deduced from the similarity in the ultimate chemical composition of Granites, Felsites, Rhyolites, and Obsidians, as shown by the following table of analyses, in which their mean composition is given :— Granite Tongues and Veins.-Similar conclusions may be deduced from a study of the tongues and veins which proceed from large masses of granite and penetrate the surrounding rocks. Granite veins, when traced from the parent mass, generally pass into a fine-grained rock, which is some variety of micro-granite, felsite, or eurite, commonly having but little mica, and consisting almost entirely of a granular or micro-graphic mixture of quartz and orthoclase. In other cases they lose both quartz and mica, and appear to take up other elements from the rocks which they penetrate, so as to become very different in mineralogical character from the central portions of the parent mass. An excellent instance of the passage of granite into a more basic rock has been described by Professor Haughton. Near Carlingford, in county Louth, there is a granitic tract about five miles in diameter, and the composition of the main mass is as follows: Quartz. 20.70 66.37 12.76 99.83 This granite sends veins into the overlying Carboniferous limestone, and the contact of the two rocks-igneous and aqueous-has greatly altered the characters of both. The limestone is converted into a bluish sugary marble contain |