holes have then been covered by the next stratum, and so preserved for all time, like the ripple- and current-marks, which are also of frequent occurrence on the surfaces of some sedimentary rocks.-Vide Froriep's Neue Notizen, 1839, vol. xi. p. 134; Ann. d. Sc. géol. 1843, p. 61; Compt. rend. 1861, vol. 53, p. 649; Lyell in Royal Institution of Great Britain, 1851-4, Cepr. und Geologie (translated 1858), i. p. 390; i. p. 150. Animals also take part in the transformation of rock surfaces. Certain kinds of mollusca on the sea coast (Pholades) have the peculiar habit of burrowing several inches deep into limestones or dolomite rocks, and even into clays as well as much harder rocks; (as, for instance, mica-schist), so as by degrees to perforate the whole surface. Ancient lines of coast are sometimes to be recognised by means of their appearance. Rounded Stones, Gravel, Shingle, Pebbles, or Boulders.— These stones have usually been wholly or partially rounded by the action of water. There are, however, such as have been rounded by the motion of glaciers, and some even appear to have been rounded in clefts of rocks, the sides of which have been much agitated. There are also some special points respecting them which deserve attention. In the first place most pebbles are not spherical, but flattened and lenticular or elongated, egg-shaped, &c. This very universal law is evidently the result of an unequal degree of resistance to waste presented by the stone in the direction of one or more normal axes. In the case of rocks of slaty texture or the like, this phenomenon may be readily conceived; but in the case of compact or granular rocks without a trace of fissile or laminated texture, it is more remarkable, and points to some parallelism of texture or structure which has hitherto escaped observation. The boulders or pebbles formed by glaciers sometimes exhibit grooves or scratches on their surface. At the foot of the Alps in the neighbourhood of Vienna, many pebbles and boulders have been formed showing deep grooves and forcible impressions, and some which are partially broken and pieced together again. In some conglomerates (as in the Nagelflue of St. Gall) pebbles are found partly forced into each other (these are EXTERNAL STRUCTURE.-JOINTED STRUCTURE. 103 usually of limestone), and in other conglomerates, for intance at Waldenburg in Silesia, there are pebbles which have been cleft asunder, their several parts somewhat displaced, and so cemented together again. But perhaps the most remarkable of these phenomena are the dolomitic limestone pebbles in a conglomerate at St. Lauretta in the Leitha mountains, many of which are hollow. Much has been written on these peculiar forms and Württenberger in von L. & Br. Jahrb. 1859, p. 153. Gurlt, ibid. 1861, p. 225. Berggeist, 1860, p. 382. Sorby, On the Direct Correlation of Mechanical and Chemical f те a le, JOINTED STRUCTURE. All large masses of rock are internally cleft by fissures or joints, and thereby divided into solids of different size and form. The general cause of this jointed structure of the mass is evidently contraction which, in the case of the igneous rocks, in all probability took place during cooling; in the case of the sedimentary during the process of their drying; and in the case of the metamorphic, which they inherited from the sedimentary, or which was renewed during the process of metamorphism. In most rocks the jointing is irregular, dividing the 1: rock into irregular masses; frequently, however, a certain degree of regularity is exhibited-i.e. the dividing fissures observe one or more prevailing directions, and are at definite distances from each other, so as to form a severance into tolerably regular plates, columns, parallelopipeds, or spherical masses. re nes na, ng are This so-called jointed structure deserves to be here described with some particularity, although it has no connection with the mineralogical composition of the rock, and solely results from the circumstances attending its original formation, and especially its solidification. Tabular Jointing.-The rock's mass is split into parallel plates or tables, and these, unlike flagstones or strata, have not been successively deposited one over the other, but were all formed simultaneously and subsequently to the first formation of the rock. This constitutes, indeed, the characteristic distinction between stratification and jointing the former being the result of successive superposition, the latter of the splitting of a previously formed mass. Tabular jointing occurs most frequently in the igneous rocks, less frequently also in the sedimentary and metamorphic. A modification of the tabular structure sometimes occurs, consisting in a curvature of the individual plates, which are frequently very thin. This is called in German Schalige absonderung, 'conchoidal jointing.' Columnar, Subcolumnar, Prismatic Jointing. The rock's mass is split into columns of from 3 to 9 faces, usually 5 or 6 faces, and the thickness of the pillars in each place is tolerably uniform, but in different places varies from a few inches to several feet. The length of the columns is of course unequal. Some are known more than 200 feet long. These columns are, however, usually cross-jointed-i.e. split into shorter blocks by means of cross courses or horizontal fissures at right angles with the first set of joints. This jointing is regular or irregular, it sometimes exhibits rounded surfaces, indicating in that case that the pillars were formed by the joining together of spherical masses (as may be clearly seen, indeed, in some places). Columnar jointing may be observed with peculiar frequency and beauty in basalt, but it also occurs in diabase, diorite, aphanite, and quartz-porphyry; less frequently in trachyte, granite, or syenite. In all these rocks this jointing is evidently the result of a special process of cooling; moreover, the axes of the columns are for the most part at right angles with the plane of the larger cooling surface. In lava streams, for instance, perpendicular to their surface; in veins or dykes of basalt, perpendicular to the walls of the cleft. If the larger cooling surface has been curviform, the columns at right angles to it will be found bent or radiating. But sedimentary rocks sometimes exhibit the phenomenon of columnar jointing. In them it is probably owing to having dried more rapidly from one side of the mass, and in rare cases, locally, to the effect of heat from contact with igneous rocks. Parallelopipedic, Cuboidal, or Rhomboidal Jointing.The rocks are severed by joints which traverse them in planes of three different directions, which, if they cross each other at right angles, produce cubes or rectangular parallelopipeds; if at inclined angles, rhomboidal solids. In sedimentary rocks the direction of one of these planes is frequently determined by the bedding, but in igneous rocks all three sets of joints are independent of such influence. Spherical, Globular, or Spheroidal Jointing. Some rocks are entirely composed of spherical masses, the interstices or spaces originally existing between them being now filled with a mass of similar substance and composition, but so that the jointing is still apparent. These spherical masses are often formed of concentric layers, and sometimes ranged over each other in columns. In the latter case the globular and columnar jointing may be said to be combined. A modification of the spherical structure is what is called ball and socket jointing, in which single masses with rounded heads more or less approach the globular shape, and seem to fit into a cavity on the other side of the fissure. This passes over into irregular or massive jointing, which occurs more or less distinctly in rocks of the most different description. All jointing becomes much more distinctly apparent when the rock is weathered, and it sometimes even appears as if the structure were solely caused by decay of the rock. Nevertheless, it is very probable that even in these cases a disposition to the severance previously existed. STRATIFICATION OF ROCKS. We have already spoken of the lamination of shaly rocks as consisting of a structure dividing those rocks in planes parallel to their bedding, and originating in the mode in which they were formed-i. e. by successive layers of deposit. 6 The same phenomenon on a larger scale is called stratification, and the individual members of the series are termed strata. Page observes in his Adv. Text Book, Thus' (speaking of stratified rocks), the terms stratum and bed are used when the deposit is of considerable thickness; layer or band when it is thin, and holds a subordinate place among the other beds; and seam when a rock of a peculiar character occurs at intervals among a series of strata. The miner, for example, speaks of a seam of coal occurring among strata of clay and sandstone, and of a band of ironstone occurring in a bed of shale.' The horizontal line on the surface of strata is termed the strike, and their steepest inclination towards the horizontal plane is termed the dip. Stratification is exhibited more especially and distinctly in the sedimentary rocks, but it is also frequently to be recognised in the metamorphic, and even the igneous rocks may exceptionally be actually stratified, if, for instance, successive streams of lava have overflowed each other, each consolidating separately. SHAPE AND BEDDING* OF ROCK MASSES. Both the shape and the mode of bedding of rock masses are dependent on the mode of their original formation. Igneous rocks neither exhibit any certain shapes nor any uniform bedding in relation to other rocks, whereas in the case of sedimentary rocks and their offspring, the metamorphic, both shape and bedding have some relation to certain general laws. The form assumed by igneous rocks depends to some extent on the shape and size of the opening by which they forced their passage from the interior of the earth. They accordingly fill clefts more or less regular in form, 6 * The word 'BEDDING' is used indifferently throughout this work in speaking of all rocks, whether stratified or not. It is taken as the equivalent of the German Lagerung. We are aware that in England this has not been always usual; nevertheless, some general word must be adopted. Mode of occurrence,' 'position,' 'lie,' &c., are all expressions which fall short of the idea intended to be conveyed.— TRANSLATOR. |