Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/321

Rh IGNEOUS Rocks] spends on the whole with the strike of the rocks. . It is. however, independent of bedding. Among curved rocks the cleavage planes may be seen traversing the contortions without sensible deﬂexion from their normal direction, parallelism, and high angle. Mr J ukes pointed out that over the whole of the south of Ireland the trend of the cleavage seldom departs 10° fron1 the normal direction E. 2 N., no matter what may be the differences in character and age of the rocks which it crosses. Some of the more obvious characters of cleavage are shown in ﬁg. 54, which represents a block of cleaved variegated slate about 18 inches in height. The left side of the block which is in shadow is formedbya smooth cleavage plane, and the whole block might be split into laminae parallel to that plane as shown by the clea'-age lines in front. The lines of stratification are marked by the white and dark contorted bands, the axes of which evidently correspond nearly with the direction of the cleavage. These bands are commonly marked in nature by zones of different colour, and sometimes of texture. In the present instance the white bands are more sandy than the rest of the mass, and the cleavage-planes only partially enter them. This specimen is further interesting as it bears witness by its puckered bedding to the great lateral pressure in virtue of which, as we have already seen (ante, p. 261), the cleavage structure has been produced. VII. Iu_'EoL's ROCKS As P- 'r 01-‘ THE STRUCTURE 01-‘ THE E.-xr:TI1‘s CRUST. In this section we shall consider the part taken by igneous rocks in the architecture of the earth’s crust. Their litho- logical diﬂferences having already been described in part ii., it is their larger features in the ﬁeld that now require attention,—features which in some cases can be well illus- trated by reference to the action of modern volcanoes, and in other cases bring before us parts of the economy of vol- canoes which can never be reached in any recent cone. A study of the igneous rocks of former ages thus serves to augment our knowledge of volcanic action. At the outset an obvious distinction must be drawn between those igneous masses which reached the surface and consolidated there, like modern lava streams or showers of ashes, and those which we must believe never found their way to the surface but consolidated at a greater or less depth beneath it. There must be the same division to be drawn in the case of every active volcano of the present day. But we can examine only the materials which reach the sur- face, and we can but speculate as to the nature and arrange- ment of what still lies underneath. In the revolutions to which the crust of the earth has been subjected, however, the subterranean continuations of volcanic sheets have often been laid bare, and not only so, but sections have been opened into the very heart of masses which, though molten and eruptive, seem never to have been directly connected with actual volcanic outbursts. All those subterranean intruded masses, whiclrare now revealed at the surface only after the removal of the depth of rock which once covered them, may be grouped together into one division under the names plutonir-, z'm.'.rusz've, or subsequent. On the other hand, all those which came up to the surface as ordinary volcanic rocks, whether molten or fragmental, and were consequently contemporaneously interstratiﬁed with the formations which happened to be in progress on the surface at the time, may be classed in a second group under the names volcanic, interbeclded, or c0n(empo7'a7zeous. _ It is obvious of course that these are only relative terms. Every truly volcanic mass which, by being poured out as a lava-stream at the surface, came to be regularly inter- stratiﬁed with contemporaneous accumulations, must have been directly connected below with molten matter which GEOLOGY 37 did not reach the surface. One part of the total mass therefore would be included in the second group, while another portion, if ever exposed by geological revolutions, would be classed with the first group. Seldom, however, can the same masses which ﬂowed out at the surface be traced directly to their original underground prolonga- tions. It is evident that an intrusive rock, though necessarily subsequent in age to the rocks through which it has been thrust, need not be long subsequent. Its relative date can only be certainly afﬁrmed with reference to the rocks through which it has broken. It may be older than other rocks through which it has not been intruded but which lie almost immediately above it. The probable geological d.1te of its eruption must be decided by the evi- dence to be obtained from the grouping of the rocks all around. Its intrusive character can only certainly deter- mine the limit of its antiquity. We know that it must be younger than the rocks it has invaded ; how much younger must be otherwise determined. On the other hand, an in- terbedded or contemporaneous igneous rock has its date precisely ﬁxed by the geological horizon on which it lies. A lava-bed or tuff intercalated among strata containing Sp/zenopteris qﬂizcis, Lepidocleml-ron vellheimimzmn, Leper- clitia, and other associated fossils, would unequivocally prove the existence of volcanic action at the surface during the Lower Carboniferous period, and at that particular part of the period represented by the horizon occupied by the volcanic bed. An interbedded and anintrusive mass found on the same platform of strata would not necessarily be coeval. On the contrary, the latter, if clearly intruded along the horizon of the former, would necessarily be posterior in date. It will be understood then that the two groups have their respective limits determined solely by their relations to the rocks among which they may happen to lie. The value of this classiﬁcation for geological purposes is great. It enables the geologist to place and consider by themselves the granites, quartz-porphyries, and other crys- talline masses which, though lying sometimes perhaps at the roots of ancient volcanoes, and therefore intimately con- nected with volcanic action, yet owe their special characters to their having consolidated under pressure at some depth within the earth’s crust; while he arranges in another series the lavas and tuffs which, thrown out to the surface, bear the closest resemblance to the ejected materials from modern volcanoes. He is thus presented with the records of hypogene igneous action in the one group, and with those of superﬁcial volcanic action in the other. He is furnished with a method of chronologically arranging the volcanic phenomena of past ages, and is thereby enabled to collect materials for a history of volcanicaction over the globe. In adopting this classiﬁcation for unravelling the geologi- cal structure of a region where igneous rocks abound, the geologist will encounter instances where it may be diﬂicult or impossible to decide in which group a particular mass of rock must be placed. He will bear in mind, however, that after all, such schemes of classiﬁcation are proposed only for convenience in systematic work, and that there are no corresponding hard and fast lines in nature. He will recog-- nize that all crystalline or glassy igneous rocks, whether the portion visible be iuterbedded or intrusive, must be intru- sive at a greater or less depth from the surface. Every contemporaneous sheet has proceeded from seine internal pipe or mass, so that though interbedded and contemporane- ous with the strata at the top, it is intrusive in relation to the strata below. But we cannot always assert that an intrusive mass must have been connected with an outﬂowing interbedded sheet above. Seclion I.—Plutonic, I ntrzI.sz'—ve, or Subsequent I [/neous Rocks. Under this section we have to consider the part played