Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/325

Rh PLUTONIC IGNEOUS R0cKs.] because they have presented lines of more easy escape for the igneous matter pressed from below. The molten rock has very generally insinuated itself along the coal-seams, sometimes taking the lower, sometimes the upper surface, and not infrequently forcing its way along the centre. In the destruction or alteration of coal and bituminous shales a process of subterranean distillation must often have been set in progress. The gases evolved would ﬁnd their way to the surface through joints and pores of the overlying rock. The liquid products, on the other hand, would be apt to collect in ﬁssures and cavities. In central Scotland, where the coal-fields have been so abundantly pierced by igneous masses, petrolemn and asphaltnm are of frequent ' occurrence in many districts, sometimes in chinks and veins of sandstones and other sedimentary strata, sometimes in the cavities of the igneous rocks themselves. It is a remarkable fact that, striking as is the change produced by the intrusion of basalt into coals and l_-itumin- ~_ ous shalcs, it is hardly more conspicuous than the alteration eflectctl on the invading masses themselves. A compact crystalline black heavy basalt or dolerite, when it sends sheets and veins into a coal or highly carbonaceous shale, becomes yellow or white, earthy, and friable, loses weight, ceases to have any apparent crystalline texture, and in short passes into what any observer would at first unhesitatingly pronounce to be a mere clay. It is only when the distinctly intrusive character of this substance is recognized in_ the veins and ﬁngers which it sends out, and in its own irregu- lar course in the coal, that its true nature is made evident (see ﬁg. llicroscopical examination shows that this “ white-rock” or “ white—trap” is merely an altered form of basalt, wherein the felspar crystals, though much decayed, can yet be traced, the augite, olivine, and magnetite being more or less completely changed into a mere pulverulent earthy substance. A specimen of this altered rock was analysed by Mr Henry with the following results :— Silica ....................................... ..38'83O H Alumina ..... ... . . . . . . . . . . . . . . . . . . . . . . . . . .1 3 ‘$250 Lime ........................................ .. 3'9'25 Magiiesiu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-180 Soda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 0 971 Potash. .................................... .. O‘-1'22 Protoxide of iron ......................... ..l3 8-30 Peroxide of iron . . . .. . . . . . . . . . . . . . . . . . . .. . -£335 Carbonic acid .............................. .. 9 320 'atcr .................................... . .1 1'01 0 100'0T3 It is evident that most of the alkalies and much of the silica have been removed, and that most of the iron exists as carbonate of the protoxidc. In connexion with the alteration produced by igneous sheets upon their contiguous stratified rocks, reference may here be made to the lithological differences traceable within the igneous masses. The close grain already referred to as characteristic of the upper and 11nder portions of an intrusive sheet evidently depends upon more rapid cooling towards the surface of contact with the adjacent cold rocks. Vhen thin slices of these marginal parts are placed under the microscope, they sometimes show abundant black microlites which disappear as the rock is traced away from the margin. They may be regarded as incipient stages in the crystallization of the magnetite, arrested in their de- velopment by the rapid consolidation of the outer parts of the rock. In the central portions they have had an opportunity of coalescing into octahedra or groups of definite isometric crystals. A series of sections of a rock, from the outer edge where the arrested crystallites occur to the centre where deﬁnitely—built crystals appear, brings in this way before us a history of the stages in the consoli- dation of the mass. But considerable differences in composition may also be GEOLOGY 311 detected in different portions of the same intrusive sheet. A rock which at one place gives under the microscope a coarsely crystalline texture with the petrographical elements of d olerite will at a short distance show abundant orthoclase and free quartz-—minerals which do not belong to normal dolerite. These differences, like those above referred to as noticeable among amorphous bosses, seem too local and sporadic to be satisfactorily referred to original differences in the composition of various parts of the molten mass, or to segregation by gravitation or otherwise. They suggest rather that the great intrusive sheets, in their passage . through the rocks underneath, have here and there involved and melted down portions of these rocks, and have thus acquired locally an abnormal composition. III. VF.1_'s AND DYKEs.—Veins of igneous rock may occur indiffcrently in igneous, aqueous, or metamorphic rocks. They may range in diameter from mere thread-like ﬁlaments up to huge bands many feet or yards broad. In regard to their origin they may be grouped into two series —-( 1) veins of segregation, and veins of intrusion. l'ez'12s of Sag;-egatio2z..—Tl1ese include most of what were formerly and not very happily termed “contempor- aneous veins.” They are peculiar to crystalline rocks. They abound in many granites, likewise in some gneisses and schists. They may not infrequently be observed in sheets of diorite, dolerite, and diabase. They run as straight, curved, or branching ribands, seldom exceeding -.1 foot in thickness. Most frequently they are finer in texture than the rock which they traverse, though nov and then the reverse is the case, more especially in granite. Close examination of them shows that they are not sharply deﬁned by a definite junction line with the enclosing rock, but that on the contrary they are welded into that rock in such a way that they cannot easily be broken along the plane of union. This welding is found to be due to the mutual pro- trusion of the component crystals of the vein and of the surrounding rock——a structure sometimes admirably re- vealed under the microscope. Veins of this kind are evi- dently to be referred to the earliest condition of the rocks in which they occur. They point to some process, still un- explained, whereby into rents formed in the deeply buried, and at least partially consolidated or possibly colloid, mass there was a transfusion or exosmosis of some of the crystal- lizing minerals. l'ez'ns of Inlr-usz°o9z.—These are portions of once—melted or at least pasty matter which have been injected into rents of previously solidified rocks. When traceable suffi- ciently far, they may be seen to swell out and merge into their large parent mass, while in the opposite direction they- may become attenuated into mere threads. Sometimes they Fm. :36_—Inn'usivc igneous rock. run for many yards in tolerably straight lines, and when this takes place along the stratification they look like beds. At these parts, they are of course really intrusive sheets. But they may frequently be found to start suddenly upward or downward, and to break across the bedding 1_n a Very irregular manner. In ﬁg. 56 t represents an lntrusive