Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/324

Rh 310 ently melting up portions of the rocks through which they ' have made their way. Such bosses have frequently tortu- ous boundary-lines, since they send out veins into or cut capriciously across the surrounding rocks. In Wales, as shown by the maps and sections of the Geological Survey, the Lower Silurian formations are pierced by huge bosses of different crystalline rocks, mostly included under the old term “ greenstonc,” which, after running for some way with the strike of the strata, turn round and break across it, or branch and traverse a considerble thickness of strati- ﬁed rock. In central Scotland numerous masses of dolerite and quartziferous (liabase have been intruded among the Lower Carboniferous formations. One horizon on which they are particularly abundant lies about the base of the Carboniferous Limestone series. Along that horizon they rise to the surface for many miles, sometimes ascending or descending in geological position, and breaking here and there abruptly across the strata. There can be little doubt that they have actually melted down some parts of the stratiﬁed rocks, particularly the limestone. Considerable petrographical differences occur among them which may perhaps be in some measure due to the incorporation of such extraneous material int.o their mass. Gaps occur where these intrusive rocks do not rise to the surface, but as they resume their position again not far off, it may be presumed that they are really connected under these blank intervals. The amount and nature of the alteration produced on contiguous rocks by the invasion of an intrusive boss vary necessarily with the character and bulk of the igneous mass, as well as with the susceptibility of the surrounding rock to metamorphism. Induration is generally traceable; shales are hardened into porcelain, jasper, Lydian—stone, or some other ﬂinty argillaceous rock. Sandstones are con- verted into a kind of lustrous quartz-rock. Limestones are made to assume a granular or crystalline texture, passing into marble or sometimes into dolomite. Under favour- able conditions crystals ot garnet, analcime, pyrite, and other minerals are developed in the surrounding altered rock. There can be little doubt that, though the portions of these rocks now visible consolidated under a greater or less depth of overlying rock, they must in many cases have been directly connected with superﬁcial volcanic action. Some of them may have been underground ramiﬁcations of the ascending molten rock which poured forth at the surface in streams of lava. Others may mark the position of intruded masses which were arrested in their ascent in the unsuccess- ful attempt to open a new volcanic vent. II. Sui-:1-:'rs.—’I‘hese are masses of crystalline rock which have been intruded as sheets between other rocks, and now appear as more or less regularly deﬁned beds. In almost all cases it will be found that these intrusions have taken place between the planes of stratiﬁcation. The ascending mass of molten matter, after breaking across the rocks, or rather after ascending through ﬁssures either previously formed or opened at the time of the outburst, has at last found its path of least resistance to lie along the bedding planes of the strata. Accordingly it has thrust itself between the beds, raising up the overlying mass and solidifying as a nearly or exactly parallel cake or bed. _ It is evident that one of these intercalated intrusive sheets of igneous rock must present such points of rescin- blance to a truly contemporaneous bed of lava as to make it occasionally a somewhat difficult matter to determine its true character, more especially when, owing to extensive denudation, only a small portion of the rock can now be seen. The following characters mark intrusive sheets, though they must not be supposed to be all present in every GEOLOGY case. (1.) They do not rigidly conform to the bedding, but [1v. STIIUCTUII.- L. sometimes break across it and run along on another plat- form. 3.) They catch up and involve portions of the sur- rounding strata. (3.) They are connnonly most close-grained at their upper and under surfaces, and most coarsely crystal- line in the central portions. (4.) They are very rarely cellular or amygdaloidal. (5.) The rocks both above and below them are usually hardened and otherwise more or less altered. Many of the older volcanic rocks occur in this form, as felstonc, quartz-porphyry, diorite, melaphyre, diabase, dolerite, basalt, and others. The rcnmrlts above made regarding the connexion of intrusive bosses with volcanic action may be repeated with even greater definitcliess here. Intrusive sheets abound in old volcanic districts intimately associated with dykes and surface outﬂows, and thus bring- ing before our eyes traces of the underground mechani.-m of the volcanoes. - The same kinds of alteration may be observed along the line of junction of intrusive sheets with the adjacent rock '. as in the ease of amorphous masses; but as the boundary lines are often very sharply deﬁned they present the process of alteration in a more generally accessible and interesting form. S:m.'lst011c, for example, besides being induratcd and acquiring the distinct lustre of quartzite, may occasion- ally be seen to possess a distinctly prismatic structure—the prisms or columns diverging at right angles to the line of junction with the igneous rock. Even microscopic black microlites, like those which occur in basalt-roelcs, have been detected in altered sandstone, in the minute fissures of which they may be supposed to have been sublimed from the molten injected mass. Argillaceous rocks are commonly converted into hard ﬂinty textures to which the names of ﬂint.y-slate, Lydian—stone, jasper, and porcellanite have been applied. Coal-seams when invaded by intruded sheets of igneous matter assume different aspects according to the thickness and nature of the invading sheet, the ti.-ptli of the coal—seam, and probably to other less easily recognizable eauses. In some cases the coal has been fused and has acquired a blistered or vesicular texture, the gas cavities being either empty or ﬁlled with mineral matter such as calcite. In other cases it has nearly disappeared, the re- maining portion being a black soot or ash. In others it has become hard and brittle, and has been converted into a kind of anthracite or “ blind—eoal,” owing to the loss of its more volatile portions. In the Ayrshire coal—ﬁelds the coal seams have sometimes become beautifully columnar owing to the intrusion of a sheet of basalt along them. The hexagonal and pentagonal columns diverge like rows of stout pencils from the surfaces of the basalt. In one coal- ﬁeld of that county a seam of coal has been converted into graphite. The accompanying section (ﬁg. by the late I-‘Ia. 5.3-.—Sheets and strings of intrusive rock in the Ten-yard C rut, South Staftbrdshirc. M r J ukcs represents one of the numerous sheets of “ white rock ” intruded into the South Staﬂbrdslrire coal—ﬁeld. The horizontal distance shown in this section is more than 100 yards. The coal (b) resting on sandstone (z-) is traversed b irrecrular strings and sheets ((1) of what the miners term “)ivlritE::>-rock,” which proceed from the large basalt masses of the district. The coal has there become dull and anthra- eitic, and is not worth being extracted. When a coal-ﬁeld is much invaded by igneous rocks the seams of coal are usually found to have suffered more than the other strata, not merely because they are specially liable to alteration from the proximity of heated surfaces, but