300 the inclined position of strata makes them cover more horizontal space; in the present instance it increases that space to 1200 feet. A convenient rule was given many years ago by the late Mr Charles Maclaren of Edinburgh for estimating the thickness of strata inclined at angles of less than -15°. The real thickness of a mass of inclined strata is —,‘,—_.th of its ap- parent thickness for every 5’ of dip. Thus if a set of beds dip steadily in one direction at 5° for a horizontal space of 1200 feet measured across the strike, their actual thickness will be T‘.~_.th or 100 feet. If the dip be 15°. the true thick- ness will be -fifths or 400 feet, and so on. IV. CURVATURES or Rocxs. A little refiexion will show that though, so far as regards the trifling portions of the rocks visible at the surface, we might regard the inclined surfaces of the strata as parts of straight lines, they must nevertheless be parts of large curves. Take, for example, the section given in fig. 19. At the north end of that section we observe the beds to plunge one after another into the earth at an angle of 35°. By degrees the inclination increases until it reaches 50°. As there is no dislocation or abrupt change of angle, but a gradual transition, it is evident that the beds at the north end cannot proceed indefinitely downward at the same angle which they have at the surface, but must bend round to accommodate themselves to the higher inclination which sets in southwards. By prolonging the lines of the beds for some way beneath the sea—level, we can show graphically the nature of the curve. In every instance therefore where, in walking over the surface, we traverse a series of strata which gradually, and without dislocations, increase or diminish in inclination, we cross part of a great curva- ture in the strata of the earth’s crust. Such foldings, however, can often be distinctly seen, either on some cliff or coast-line, or in the traverse of a piece of hilly or mountainous ground. The observer can- not long continue his researches in the field without dis- covering that the rocks of the earth’s crust have been almost everywhere thrown into curves, usually so broad and gentle as to escape observation except when specially looked for. The outcrop of beds at the surface is commonly the trunca- tion of these curves. The strata must once have risen above the present surface, and in many cases may be found descending to the surface again with a contrary dip, the intervening portion of the undulation having been worn away. If then the inclination of rocks is so closely connected with their curvature, a corresponding relation must hold between their strike and curvature. In fact, the prevalent s.rike of a region is determined by the direction of the axes of the great folds into which the rocks have been thrown. If the curves are gentle and inconstant there will be a cor- responding variation in the strike. But should the rocks be strongly plicated, there will necessarily be the most thorough coincidence between the strike and the direction of the plieation. The curvature occasionally shows itself among horizontal or gently inclined strata in the form of an abrupt inclina- tion, and then an immediate resumption of the previous flat or sloping character. The strata are thus bent up and continue on the other side of the tilt at a higher level. Such bends are called monoclines or monoclinal folds, because they present only one fold, or one half of a fold, instead of the two which we see in an arch or trough. The most notable instance of this structure in Britain is that of the Isle of Wight, of which a section is given in fin’. 20. The Cretaceous rocks on the south side of the island rapidly rise in inclination till they become nearly vertical. GEOLOGY [1v. STRUCTFRA L. The Lower Tertiary strata follow with a similar steep (lip, but rapidly flatten down towards the north coast. Some FIG. 20.—Scction of the Isle of 'i;:ht—a monnclinal curve. (1. Chalk; (2, Wool- wieh and ltrading bi.-«ls; r. London clay; (I, lingshot series; 1-, lleadon series; f, y, Osborne and Ueinbridgc series. remarkable cases of the same structure have been brought to light by Mr J. W. Powell in his survey of the Colorado region. It much more frequently happens that the strata lltl'C been bent into arches and troughs, so that they can be seen dipping under the surface on one side of the axis of a fold, and rising up again on the other side. Where they dip away from the axis of movement the structure is termed an unticliue or a-nticlinal fold ; where they dip towards the Fro. 21.—I‘lan of antielinai and s_'nelin:rl folds. axis, it is a s_2/mrline or sg/nclimrlfold. The diagram in fig. 21 may be taken to represent a series of strata (1-17) thrown into an anticline (AA') and syncline (DE). A sec- tion drawn across these folds in the line CD would sl1o'.‘
__,
Sn.-rian on Ilrlf 6' D. ,/ FIG. 22.—Section of antcilinal and s_-nclinal folds on the line CD (fig. 21). the structure given in fig. 22. Here we see that, at the part of the anticlinal axis (A) where the section crosses, be-.l N 0. 4 forms the crown of the arch, Nos. 1, 2, and 3 being concealed beneath it-. On the east side of the axis th: strata follow each other in regular succession as far as No. 13, which, instead of passing here under the next in order, turns up with a contrary dip and forms the centre of a trough or syncline (B). From underneath No. 13 on the east side, the same beds rise to the surface which passed beneath it on the west side. The particular bed marked EF has been entirely removed by denudation from the top of the anticline, and is buried deep beneath the centre of the syncline. Such foldings of strata must always die out unless they are abruptly terminated by dislocations. In the cases given in fig. 21, both the arch and trough are represented as diminishing, the former towards the north, the latter
towards the south. The observer in passing northwards