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diff --git a/old/3772-h/files/ch34.html b/old/3772-h/files/ch34.html new file mode 100644 index 0000000..303eda5 --- /dev/null +++ b/old/3772-h/files/ch34.html @@ -0,0 +1,466 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 588">[ 588 ]</a></p> + +<p> </p> + +<center><b>Chapter XXXIV</b><br> +<br> +METAMORPHIC ROCKS—<i>continued.</i></center> + +<p class="intro">Definition of slaty Cleavage and Joints. — +Supposed Causes of these Structures. — Crystalline Theory of +Cleavage. — Mechanical Theory of Cleavage. — +Condensation and Elongation of slate Rocks by lateral Pressure. +— Lamination of some volcanic Rocks due to Motion. — +Whether the Foliation of the crystalline Schists be usually +parallel with the original Planes of Stratification. — +Examples in Norway and Scotland. — Causes of Irregularity in +the Planes of Foliation.</p> + +<p>We have already seen that chemical forces of great intensity +have frequently acted upon sedimentary and fossiliferous strata +long subsequently to their consolidation, and we may next inquire +whether the component minerals of the altered rocks usually arrange +themselves in planes parallel to the original planes of +stratification, or whether, after crystallisation, they more +commonly take up a different position.</p> + +<p>In order to estimate fairly the merits of this question, we must +first define what is meant by the terms cleavage and foliation. +There are four distinct forms of structure exhibited in rocks, +namely, stratification, joints, slaty cleavage, and foliation; and +all these must have different names, even though there be cases +where it is impossible, after carefully studying the appearances, +to decide upon the class to which they belong.</p> + +<p><b>Slaty Cleavage.</b>—Professor Sedgwick, whose essay +“On the Structure of large Mineral Masses” first +cleared the way towards a better understanding of this difficult +subject, observes, that joints are distinguishable from lines of +slaty cleavage in this, that the rock intervening between two +joints has no tendency to cleave in a direction parallel to the +planes of the joints, whereas a rock is capable of indefinite +subdivision in the direction of its slaty cleavage. In cases where +the strata are curved, the planes of cleavage are still perfectly +parallel. This has been observed in the slate rocks of part of +Wales (see Fig. 624), which consists of a hard greenish slate. The +true bedding is there indicated by a number of parallel stripes, +some of a lighter and some of a darker colour than the general +mass. Such stripes are found to be parallel to the true planes of +stratification, wherever these are manifested by ripple-mark or by +beds</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 589">[ 589 ]</a></p> + +<center><img src="../images5/fig624.jpg" width="331" height="102" alt= +"Fig. 624: Parallel planes of cleavage intersecting curved strata."> +</center> + +<p>containing peculiar organic remains. Some of the contorted +strata are of a coarse mechanical structure, alternating with +fine-grained crystalline chloritic slates, in which case the same +slaty cleavage extends through the coarser and finer beds, though +it is brought out in greater perfection in proportion as the +materials of the rock are fine and homogeneous. It is only when +these are very coarse that the cleavage planes entirely vanish. In +the Welsh hills these planes are usually inclined at a very +considerable angle to the planes of the strata, the average angle +being as much as from 30° to 40°. Sometimes the cleavage +planes dip towards the same point of the compass as those of +stratification, but often to opposite points.* The cleavage, as +represented in Fig. 624, is generally constant over the whole of +any area affected by one great set of disturbances, as if the same +lateral pressure which caused the crumpling up of the rock along +parallel, anticlinal, and synclinal axes caused also the +cleavage.</p> + +<img src="../images5/fig625.jpg" width="282" height="186" alt= +"Fig. 625: Section in Lower Silurian slates of Cardiganshire, showing the cleavage planes bent along the junction of the beds." + align="right"> + +<p>Mr. T. McK. Hughes remarks, that where a rough cleavage cuts +flag-stones at a considerable angle to the planes of +stratification, the rock often splits into large slabs, across +which the lines of bedding are frequently seen, but when the +cleavage planes approach within about 15° of stratification, +the rock is apt to split along the lines of bedding. He has also +called my attention to the fact that subsequent movements in a +cleaved rock sometimes drag and bend the cleavage planes along the +junction of the beds in the manner indicated in Fig. 625.</p> + +<p><b>Jointed Structure.</b>—In regard to joints, they are +natural</p> + +<p class="fnote">* Geol. Trans., 2nd series, vol. iii, p. 461.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 590">[ 590 ]</a></p> + +<p>fissures which often traverse rocks in straight and +well-determined lines. They afford to the quarryman, as Sir R. +Murchison observes, when speaking of the phenomenon, as exhibited +in Shropshire and the neighbouring counties, the greatest aid in +the extraction of blocks of stone; and, if a sufficient number +cross each other, the whole mass of rock is split into symmetrical +blocks. The faces of the joints are for the most part smoother and +more regular than the surfaces of true strata. The joints are +straight-cut chinks, sometimes slightly open, and often passing, +not only through layers of successive deposition, but also through +balls of limestone or other matter which have been formed by +concretionary action since the original accumulation of the strata. +Such joints, therefore, must often have resulted from one of the +last changes superinduced upon sedimentary deposits.*</p> + +<center><img src="../images5/fig626.jpg" width="344" height="194" alt= +"Fig. 626: Stratification, joints, and cleavage."></center> + +<p>In Fig. 626 the flat-surfaces of rock, A, B, C, represent +exposed faces of joints, to which the walls of other joints, J J, +are parallel. S S are the lines of stratification; D D are lines of +slaty cleavage, which intersect the rock at a considerable angle to +the planes of stratification.</p> + +<p>In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, +enormous masses of limestone are cut through so regularly by nearly +vertical partings, and these joints are often so much more +conspicuous than the seams of stratification, that an inexperienced +observer will almost inevitably confound them, and suppose the +strata to be perpendicular in places where in fact they are almost +horizontal.†</p> + +<p>Now such joints are supposed to be analogous to the partings +which separate volcanic and Plutonic rocks into cuboidal and +prismatic masses. On a small scale we see clay and starch when dry +split into similar shapes; this is often caused by simple +contraction, whether the shrinking be due</p> + +<p class="fnote">* Silurian System, p. 246.<br> +† Introduction to Geology, chap. iv.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 591">[ 591 ]</a></p> + +<p>to the evaporation of water, or to a change of temperature. It +is well known that many sandstones and other rocks expand by the +application of moderate degrees of heat, and then contract again on +cooling; and there can be no doubt that large portions of the +earth’s crust have, in the course of past ages, been +subjected again and again to very different degrees of heat and +cold. These alternations of temperature have probably contributed +largely to the production of joints in rocks.</p> + +<p>In many countries where masses of basalt rest on sandstone, the +aqueous rock has, for the distance of several feet from the point +of junction, assumed a columnar structure similar to that of the +trap. In like manner some hearth-stones, after exposure to the heat +of a furnace without being melted, have become prismatic. Certain +crystals also acquire by the application of heat a new internal +arrangement, so as to break in a new direction, their external form +remaining unaltered.</p> + +<p><b>Crystalline Theory of Cleavage.</b>—Professor Sedgwick, +speaking of the planes of slaty cleavage, where they are decidedly +distinct from those of sedimentary deposition, declared, in the +essay before alluded to, his opinion that no retreat of parts, no +contraction in the dimensions of rocks in passing to a solid state, +can account for the phenomenon. He accordingly referred it to +crystalline or polar forces acting simultaneously, and somewhat +uniformly, in given directions, on large masses having a +homogeneous composition.</p> + +<p>Sir John Herschel, in allusion to slaty cleavage, has suggested +that “if rocks have been so heated as to allow a commencement +of crystallisation—that is to say, if they have been heated +to a point at which the particles can begin to move among +themselves, or at least on their own axes, some general law must +then determine the position in which these particles will rest on +cooling. Probably, that position will have some relation to the +direction in which the heat escapes. Now, when all, or a majority +of particles of the same nature have a general tendency to one +position, that must of course determine a cleavage-plane. Thus we +see the infinitesimal crystals of fresh-precipitated sulphate of +barytes, and some other such bodies, arrange themselves alike in +the fluid in which they float; so as, when stirred, all to glance +with one light, and give the appearance of silky filaments. Some +sorts of soap, in which insoluble margarates* exist,</p> + +<p class="fnote">* Margaric acid is an oleaginous acid, formed from +different animal and vegetable fatty substances. A margarate is a +compound of this acid with soda, potash, or some other base, and is +so named from its pearly lustre.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 592">[ 592 ]</a></p> + +<p>exhibit the same phenomenon when mixed with water; and what +occurs in our experiments on a minute scale may occur in nature on +a great one.”*</p> + +<p><b>Mechanical Theory of Cleavage.</b>—Professor Phillips +has remarked that in some slaty rocks the form of the outline of +fossil shells and trilobites has been much changed by distortion, +which has taken place in a longitudinal, transverse, or oblique +direction. This change, he adds, seems to be the result of a +“creeping movement” of the particles of the rock along +the planes of cleavage, its direction being always uniform over the +same tract of country, and its amount in space being sometimes +measurable, and being as much as a quarter or even half an inch. +The hard shells are not affected, but only those which are +thin.† Mr. D. Sharpe, following up the same line of inquiry, +came to the conclusion that the present distorted forms of the +shells in certain British slate rocks may be accounted for by +supposing that the rocks in which they are imbedded have undergone +compression in a direction perpendicular to the planes of cleavage, +and a corresponding expansion in the direction of the dip of the +cleavage.‡</p> + +<p>Subsequently (1853) Mr. Sorby demonstrated the great extent to +which this mechanical theory is applicable to the slate rocks of +North Wales and Devonshire,§ districts where the amount of +change in dimensions can be tested and measured by comparing the +different effects exerted by lateral pressure on alternating beds +of finer and coarser materials. Thus, for example, in Fig. 627 it +will be seen that the sandy bed <i>d f,</i> which has offered +greater resistance, has been sharply contorted, while the +fine-grained strata, <i>a, b, c,</i> have remained comparatively +unbent. The points <i>d</i> and <i>f</i> in the stratum <i>d f</i> +must have been originally four times as far apart as they are now. +They have been forced so much nearer to each other, partly by +bending, and partly by becoming elongated in the direction of what +may be called the longer axes of their contortions, and lastly, to +a certain small amount, by condensation. The chief result has +obviously been due to the bending; but, in proof of elongation, it +will be observed that the thickness of the bed <i>d f</i> is now +about four times greater in those parts lying in the main direction +of the flexures than in a plane</p> + +<p class="fnote">* Letter to the author, dated Cape of Good Hope, +Feb. 20, 1836.<br> +† Report, Brit. Assoc., Cork, 1843, Sect. p. 60.<br> + ‡ Quart. Geol. Journ., vol. iii, p. 87, 1847.<br> +§ On the Origin of Slaty Cleavage, by H. C. Sorby, Edin. New +Phil. Journ., 1853, vol. lv, p. 137.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 593">[ 593 ]</a></p> + +<img src="../images5/fig627.jpg" width="216" height="537" alt= +"Fig. 627: Vertical section of slate rock in the cliffs near Ilfracombe, North Devon." + align="right"> + +<p>perpendicular to them; and the same bed exhibits cleavage planes +in the direction of the greatest movement, although they are much +fewer than in the slaty strata above and below.</p> + +<p>Above the sandy bed <i>d f,</i> the stratum <i>c</i> is somewhat +disturbed, while the next bed, <i>b,</i> is much less so, and a not +at all; yet all these beds, <i>c, b,</i> and <i>a,</i> must have +undergone an equal amount of pressure with <i>d,</i> the points a +and g having approximated as much towards each other as have <i> +d</i> and <i>f.</i> The same phenomena are also repeated in the +beds below <i>d,</i> and might have been shown, had the section +been extended downward. Hence it appears that the finer beds have +been squeezed into a fourth of the space they previously occupied, +partly by condensation, or the closer packing of their ultimate +particles (which has given rise to the great specific gravity of +such slates), and partly by elongation in the line of the dip of +the cleavage, of which the general direction is perpendicular to +that of the pressure. “These and numerous other cases in +North Devon are analogous,” says Mr. Sorby, “to what +would occur if a strip of paper were included in a mass of some +soft plastic material which would readily change its dimensions. If +the whole were then compressed in the direction of the length of +the strip of paper, it would be bent and puckered up into +contortions, while the plastic material would readily change its +dimensions without undergoing such contortions; and the difference +in distance of the ends of the paper, as measured in a direct line +or along it, would indicate the change in the dimensions of the +plastic material.”</p> + +<p>By microscopic examination of minute crystals, and by</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 594">[ 594 ]</a></p> + +<p>other observations, Mr. Sorby has come to the conclusion that +the absolute condensation of the slate rocks amounts upon an +average to about one half their original volume. Most of the scales +of mica occurring in certain slates examined by Mr. Sorby lie in +the plane of cleavage; whereas in a similar rock not exhibiting +cleavage they lie with their longer axes in all directions. May not +their position in the slates have been determined by the movement +of elongation before alluded to? To illustrate this theory some +scales of oxide of iron were mixed with soft pipe-clay in such a +manner that they inclined in all directions. The dimensions of the +mass were then changed artificially to a similar extent to what has +occurred in slate rocks, and the pipe-clay was then dried and +baked. When it was afterwards rubbed to a flat surface +perpendicular to the pressure and in the line of elongation, or in +a plane corresponding to that of the dip of cleavage, the particles +were found to have become arranged in the same manner as in natural +slates, and the mass admitted of easy fracture into thin flat +pieces in the plane alluded to, whereas it would not yield in that +perpendicular to the cleavage.*</p> + +<p>Dr. Tyndall, when commenting in 1856 on Mr. Sorby’s +experiments, observed that pressure alone is sufficient to produce +cleavage, and that the intervention of plates of mica or scales of +oxide of iron, or any other substances having flat surfaces, is +quite unnecessary. In proof of this he showed experimentally that a +mass of “pure white wax, after having been submitted to great +pressure, exhibited a cleavage more clean than that of any +slate-rock, splitting into laminæ of surpassing +tenuity.”† He remarks that every mass of clay or mud +is divided and subdivided by surfaces among which the cohesion is +comparatively small. On being subjected to pressure, such masses +yield and spread out in the direction of least resistance, small +nodules become converted into laminæ separated from each +other by surfaces of weak cohesion, and the result is that the mass +cleaves at right angles to the line in which the pressure is +exerted. In further illustration of this, Mr. Hughes remarks that +“concretions which in the undisturbed beds have their longer +axes parallel to the bedding are, where the rock is much cleaved, +frequently found flattened laterally, so as to have their longer +axes parallel to the cleavage planes, and at a considerable angle, +even right angles, to their former position.”</p> + +<p>Mr. Darwin attributes the lamination and fissile structure</p> + +<p class="fnote">* Sorby, as cited above, p. 741, note.<br> +† Tyndall, View of the Cleavage of Crystals and Slate +rocks.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 595">[ 595 ]</a></p> + +<p>of volcanic rocks of the trachytic series, including some +obsidians in Ascension, Mexico, and elsewhere, to their having +moved when liquid in the direction of the laminæ. The zones +consist sometimes of layers of air-cells drawn out and lengthened +in the supposed direction of the moving mass.</p> + +<p><b>Foliation of Crystalline Schists.</b>—After studying, +in 1835, the crystalline rocks of South America, Mr. Darwin +proposed the term <i>foliation</i> for the laminæ or plates +into which gneiss, mica-schist, and other crystalline rocks are +divided. Cleavage, he observes, may be applied to those divisional +planes which render a rock fissile, although it may appear to the +eye quite or nearly homogeneous. Foliation may be used for those +alternating layers or plates of different mineralogical nature of +which gneiss and other metamorphic schists are composed.</p> + +<p>That the planes of foliation of the crystalline schists in +Norway accord very generally with those of original stratification +is a conclusion long since espoused by Keilhau.† Numerous +observations made by Mr. David Forbes in the same country (the best +probably in Europe for studying such phenomena on a grand scale) +confirm Keilhau’s opinion. In Scotland, also, Mr. D. Forbes +has pointed out a striking case where the foliation is identical +with the lines of stratification in rocks well seen near +Crianlorich on the road to Tyndrum, about eight miles from +Inverarnon, in Perthshire. There is in that locality a blue +limestone foliated by the intercalation of small plates of white +mica, so that the rock is often scarcely distinguishable in aspect +from gneiss or mica-schist. The stratification is shown by the +large beds and coloured bands of limestone all dipping, like the +folia, at an angle of 32° N.E.‡ In stratified formations +of every age we see layers of siliceous sand with or without mica, +alternating with clay, with fragments of shells or corals, or with +seams of vegetable matter, and we should expect the mutual +attraction of like particles to favour the crystallisation of the +quartz, or mica, or feldspar, or carbonate of lime, along the +planes of original deposition, rather than in planes placed at +angles of 20 or 40 degrees to those of stratification.</p> + +<p>We have seen how much the original planes of stratification may +be interfered with or even obliterated by concretionary action in +deposits still retaining their fossils, as in the case of the +magnesian limestone (see <a href="ch4.html#page 63">p. 63</a>). +Hence we must expect to be frequently baffled when we attempt to +decide</p> + +<p class="fnote">* Darwin, Volcanic Islands, pp. 69, 70.<br> +† Norske Mag. Naturvidsk., vol. i, p. 71.<br> +‡ Memoir read before the Geol. Soc. London, Jan. 31, +1855.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 596">[ 596 ]</a></p> + +<p>whether the foliation does or does not accord with that +arrangement which gravitation, combined with current-action, +imparted to a deposit from water. Moreover, when we look for +stratification in crystalline rocks, we must be on our guard not to +expect too much regularity. The occurrence of wedge-shaped masses, +such as belong to coarse sand and pebbles—diagonal lamination +(<a href="ch2.html#page 42">p. 42</a>)—ripple-marked, +unconformable stratification,—the fantastic folds produced by +lateral pressure—faults of various width—intrusive +dikes of trap—organic bodies of diversified shapes, and other +causes of unevenness in the planes of deposition, both on the small +and on the large scale, will interfere with parallelism. If complex +and enigmatical appearances did not present themselves, it would be +a serious objection to the metamorphic theory. Mr. Sorby has shown +that the peculiar structure belonging to ripple-marked sands, or +that which is generated when ripples are formed during the +deposition of the materials, is distinctly recognisable in many +varieties of mica-schists in Scotland.*</p> + +<img src="../images5/fig628.jpg" width="205" height="169" alt= +"Fig. 628: Lamination of clay-stone. Montagne de Seguinat, near Gavarnie, in the Pyrenees." + align="left"> + +<p>In Fig. 628 I have represented carefully the lamination of a +coarse argillaceous schist which I examined in 1830 in the +Pyrenees. In part it approaches in character to a green and blue +roofing-slate, while part is extremely quartzose, the whole mass +passing downward into micaceous schist. The vertical section here +exhibited is about three feet in height, and the layers are +sometimes so thin that fifty may be counted in the thickness of an +inch. Some of them consist of pure quartz. There is a resemblance +in such cases to the diagonal lamination which we see in +sedimentary rocks, even though the layers of quartz and of mica, or +of feldspar and other minerals, may be more distinct in alternating +folia than they were originally.</p> + +<p class="fnote">* H. C. Sorby, Quart. Geol. Journ., vol. xix., p. +401.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch33.html"> +Chapter XXXIII</a> / <a href="ch35.html">Chapter XXXV</a></small> +</body> +</html> + |