Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/326

Rh ‘of thickness as that which is shown by veins. 312 igneous rock running through horizontal strata s. If we saw merely the horizontal portion below or above, the really igneous and intrusive nature of the rock I might escape us, but the intermediate connecting vein makes its character at once apparent. No rock exhibits so admirably as granite the varieties assumed by veins. In many cases the veins which traverse the granite itself must be regarded as segregation or inﬁltra- tion veins, as already described. But where they proceed from the granite, and traverse surrounding rocks, they are probably in most cases intrusive, though vhere granite and highly granitic gneiss are in contact we may conceive that some of the veins traversing both rocks may be segregation veins. Host large masses of granite send veins into the surrounding rocks. Frequently the veins so abound as to form a complicated net—work. They vary in breadth from several feet or even yards down to ﬁne ﬁlaments at the ends of the smaller branches. They frequently cross each other, not only outside of the granite mass, but even within it. They vary much in texture and in composition. Sometimes they are coarsely crystalline; but probably most of the veins of ' this kind are due rather to segregation than intrusion .[ost frequently granite veins traversing granite are ﬁner- grained than the main mass. Veins which are clearly in- trusive are not only ﬁner in grain than the parent granite, but sometimes present considerable differences in minera- logical composition. The mica, for example, may be reduced to exceedingly minute and not very abundant flakes, and may almost disappear. sionally assumes a subordinate place, and the rock of the veins passes into eurite, elvanite, or one of the varieties of porphyry. The rocks surrounding a granite mass and traversed by granite veins are almost always more or less metamorphosed in a belt varying in breadth from a few yards up to a mile or more. It is in this zone that the granite veins typically occur. The altered rocks have assumed the characters of gneiss, mica-schist, or other metamorphic product, but re- s'.1me their usual condition as we trace them away from the granite. Curious angular portions of them may often be observed within the granite veins and in the main granite 111153. In Cornwall the granite and surrounding slates are traversed by veins of quartz—porphyry termed elvans, which are most numerous 11ear the granite. They vary in width from a few inches or feet to 50 fathoms, their central por- tions being commonly more crystalline than the sides. In the great granite region of Leicester Mr J ukes traced some of the elvans for several miles running in parallel bands, ear-h only a few feet thick, with intervals of ‘ZOO or 300 yards between them. Many of the other intrusive rocks likewise present the phenomena of veins; diorite, diabase, Inelaphyre, and basalt furnish numerous illustrations. D3//res are wall-like masses of igneous rock, ﬁlling vertical or highly-inclined ﬁssures. They differ therefore frozn veins in the greater parallelism of their sides, their verticality, and their greater regularity of breadth and persistence of direction. They present as great a variety Some- times they occur as mere plates of rock not more than an inch or two in thickness; at other times they attain a breadth of ten or twelve fathoms. The smaller or thinner dykes can seldom be traced more than a few yards; but the larger examples may be followed sometimes for miles. Thus in the south of Scotland a remarkable series of basalt- dykes can be traced across all the geological formations of t-lnt region and even across powerful faults. They run l parallel to each other in a general north-west and south-east direction for distances of 20 and 30 miles. A remarkable dyke crosses the north of l'Inglantl from the coast of York- shire for fully 00 miles inland. GEOLOG The quartz also occa- ' Y The name dyke is applied to these masses of igneous rock on account of their resemblance to walls (Scotice, dykes). Their sides are often as parallel and perpendicular as those of a piece of masonry. .Ioreover, the resemblance to I human workmanship is sometimes brought out still more by the numerous joints which, intersecting each other along l the face of a dyke, remind us of well-ﬁtted masonry. Where the surrounding rock has decayed, the dykes may be seen projecting above the ground exactly like walls ; indeed in many parts of the west of Scotland they are made use of for enclosures. The material of the dykes has in other cases decayed, and deep ditch-like hollows are lef t to mark their sites. The coast-lines of many of the I11ner llehridcs and of the Clyde Islands furnish mnnerous admirable examples of both kinds of scenery. While veins have been injected into irregular branchin cracks, dykes have been formed by the welling upwards of liquid rock in vertical or steeply inclined ﬁssures. Some- times the line of escape has been along a fault. In Scot- land, however, which may be regarded as a typical region for this kind of geological structure, the vast majority of dykes rise along ﬁssures which have no throw, and are therefore not faults. On the contrary the dykes may be traced across some of the largest faults in the midland counties. While the term dyke might be applied to some of the wall-like intrusions of porphyry, elvanite, and even of granite, it is more typically illustrated among the augitic igneous rocks, such as basalt, diabase, &c., though also among diorites, porphyries, pitchstones, &'e. The central parts of a dyke are usually most crystalline. Towards the [1v. STl{UC'l‘L’I‘..-L. l margin the grain becomes ﬁner, and even sometimes passes into a vitreous condition. Many of the basalt dykes in different parts of Scotland are coated along the sides with a ﬁlm or crust of black vitreous tachylite. Lines of aniygdaloidal kernels may not infrequently be traced along the centre of a dyke. When the rock on one side is freshly stripped off, the dyke is usually observed to present a system of polygonal - jointing. The joints start from each face or check, and either‘ go right across to the opposite side, or branch and lose themselves about the centre. They thus divide the dyke into irregular prisms which, when the dyke is vertical, lie of course in a horizontal position, whence they depart in proportion as the dyke is inclined. Occasionally the prisms are as well-formed as in any columnar bed of basalt. A I less prominent set of joints runs parallel with the two checks of a dyke. There is usually some alteration of the stratified rocks in contact with a dyke ; but the nature and flllmllllll of the change vary within a wide range. The most sensitive material to this influence is undoubtedly coal. A seam of coal 6 or 8 feet in thickness may be observed to grow dull and brittle at a distance of 50 yards from a large dyke, becoming what is termed “blind coal,” as it then burns without ﬂame. Still nearer to the intrusive mass the coal passes into a kind of pyritous cinder scarcely half the ' original thickness of the seam. At the actual contact with the dyke it becomes by degrees a kind of caked soot not more perhaps than a few inches thick. Sandstones are hardened into a kind of lustrous quartzite, and sometimes made colunmar, shales into ﬂinty slate or porcellanite ; limestones are occasionally rendered crystalline and even dolomitic. Occasionally a segregation of new minerals has taken place in the rocks adjoining a dyke. ’»ut cases are by no means infrequent where dykes have produced little or no appreciable change upon the contiguous rocks. IV. N EcKs.—--Unrler this term are included the ﬁlled-up pipes or funnels of former volcanic vents. Every series of
 * volcanic sheets poured out at the surface must have been