Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/327

Rh VOLCANIC IGNEOUS 1:oc1<s.] connected with one or more oriﬁces which, on the cessation of the eruptions, would remain more or less completely ﬁlled with lava or with fragmentary matter. But unless sub- sequent denudation should remove the overlying cone and its surrounding piles of lava and tuff, these vents nmst remain buried under the materials which came out of them. So extensive, l1owever_,l1as been the waste of the surface in many old volcanic regions that the sites of the vents have been laid bare. In the study of these we have before us some of the more deep-seated phenomena of volcanic action never to be seen in any modern volcano. A neck is of a circular or elliptical, but occasionally of a more irregular branching form. It varies in diameter from a few yards up to a mile, or even more. It descends into the earth perpendicularly to the stratification of the formation to which it belongs. 'l‘ln1s, if a neck was formed and filled up during the accumulation of a certain group of strata, it would rise on the whole vertically through these strata, and its ejected lava or tutf would spread out con- formably among them. Should the rocks be subsequently tilted the neck would of course be thrown out of the verti- C'Ll. As a rule, however, the vertical descent of the necks into the earth’s crust has been comparatively little interfered with. The materials ﬁlling up ancient volcanic oriﬁces are sometimes crystalline, sometimes fragmental. The neck may be occupied by some form of lava, as felstone, quartz- porphyry, diabase, porphyrite, basalt ; or by the frag1nent- ary ll1:1tel'l‘Ll.5' which fell back into the throat of the volcano an-.1 finally solidified there; or by both kinds of rock com- bined. Among the Paheozoic volcanic districts of Britain the necks 11ot infrequently are ﬁlled with some siliceous cr_v.<talline rock, such as a quartz—porphyry or felstone, even where the surrounding lavas are basic. Necks of agglomer- ate and ﬁne tuﬁ’ abound among the Carboniferous and Permian volcanic regions of Scotland. The fragmentary materials consist mainly of different 1-iv_1-form rocks imbedded in a gravelly pepcrino-like matrix of more ﬁnely commiuuted debris of the same rocks; but they also contain, sometimes in abundance, fragments of the strata through which the necks have been drilled. Pieces of ﬁne stratified tuff not infrequently appear in the agglonierates. This fact, coupled with the not uncommon 0.-..-urrence of a tumultuous fracture-:1 anrl highly-inclined b;-d:ling of the materials in the necks appears to show tint the pipes were partly ﬁlled 11p by the subsidence of th3 tuﬁ‘ COl1.s'OllLl;1l‘.0(l in beds within the crater and at the upper part of the funnel. Veins of basalt abound in many of the necks of Carboniferous age in central Scotland. The strata round a neck are usually somewhat hardened. The sandstones have acquired sometimes a vitreous lustre; argillaceous beds have been indurated into porcellanite ; co'il—seams have been burnt and rendered unworkable. These changes may be due partly to the heat of the ascend- ing column of molten rock or ejected fragments, partly to the ascent of heated vapours, even for a long time subse- quently to the volcanic explosions. Proofs of a metamor- phism probably due to the latter cause may sometimes be seen within the area of a neck. It is where the altered materials are of a fragmentary character that the nature and amount of this change can be best estimated. What was originally volcanic dust has been converted into a crystalline and even porphyritic mass, through which, 11 nvever, the likewise intensely altered blocks interspersed through the agglomerate are still recognizable. Such blocks as, from the nature of their substance, must have offered most resistance to change,—pieces of sandstone or quartz, for example,—-stand out prominently in the altered mass, though even they l1ave undergone more or less modiﬁcation, the sandstone being converted into vitreous quartz—rock. GEOLOGY 313 Section I I .—- I'olcam'c, I nterbedderl, or C'onle-mporaneous Igneous Ifocks. The rocks comprised in this section have all been ejected to the surface like the lava-streams and showers of ashes of modern volcanoes. It is evident that on the whole they must agree in lithological characters with those rocks, described in the previous section, which have been extravasated by volcanic efforts though not quite reaching the surface. Yet they have some well—marked general characters, of which the most important may be thus stated. (1.) They occur as beds or sheets which conform to the bedding of the strata among which they are inter- calated. They do not break into or involve portions of the overlying beds. The upper and under por- tions of the lava sheets present commonly a scoriaceous or vesicular character, which may even be found extending throughout the whole of a sheet. (4.) Beds of tuff are frequently interstratiﬁed with the crystalline sheets. I. CRYST.LLI‘.'E.——“Tl1ll0 the underground course of a protruded mass of molten igneous rock has widely varied according to the shape of the channel through which it pro- ceeded, and in which, as in a mould, it solidiﬁed, the behaviour of the rock, once poured out at the surface, has been much more uniform. As in modern lava, the erupted rock has rolled along, varying in thickness and other minor characters, but retaining the broad general aspect of a bed or sheet. A comparison of such a bed with one of the intrusive sheets already described shows that in several important respects they differ from each other. An intrusive sheet is closest in grain near its upper and under surfaces. A con- temporaneous hed or true lava-ﬂow, on the eoutrary, is there usually most open and scoriaceous. In the one case we rarely see vesicles or amygdules, in the other they often abound. However rough the upper surface of an inter- bedded sheet may be, it never sends out veins into nor encloses portions of the snperincumbent rocks, uhich, how- ever, sometimes contain portions of it, and wrap round its hummocky irregularities, as shown in ﬁr’. 57. Occasion- Pia .57.-Section of the upper surface of an interbedded sheet T, showing how IIS imerenncsscs are wrapped round and covered by the sedimentary rocks 5. ally it may be observed to be full of rents which have been ﬁlled up with sandstone or other sedimentary material. In these cases we see that the lava cracked in solidifying, and that sand was washed into the ﬁssures where it consolidated. The amygdaloidal cavities throughout an interbedded sheet, but more especially at the top, may often be noticed with an elongated form, and even pulled out into tube-like hollows in one general direction, which was obviously the line of movement of the yet viscous mass. Some kinds of rock when occurring in interbedded sheets are apt to assume a system of columnar jointing. Basalt in particular is distinguished by the frequency and perfec- tion of its colu111ns. The Giant’s Causeway and the cliffs of Statfa, of Ardtun in Mull, and of Loch Staflin in Skye are well-known examples. The columns are set perpendicularly to the two cooling surfaces, that is, to the top and bottom of the bed. Any inclination from the horizontal in the disposition of the bed will cause a corresponding departure from the vertical on the part of the columns. Sometimes the columns are arched or curved, as in the Clam—shell Cave, 1 Staffa. X. —4o