SONS IN GEOLOGY. 



57 



so, but the rarfaoe of tho earth need not be uniformly level, even 



\V)KT<> tin- strata yet remain in t h.-n- horizontal position. Thus, in 

 J, thr rivrr, A, has worn down and carried away six layers of 

 strata, an. I is in tho oat of cutting into tho seventh. A similar 

 action has denuded tho country to the right of tho hill, c, which is 

 thus composed of strata still in their original position, and which 

 itiv found in a corresponding position on tho hill, B. After 

 <\- (Irmuliition hud hi <<n completed in tho neighbourhood of i>, 

 an upheaving force tilted tho strata, throwing them out of tlu-ir 

 position and producing a hill, by very different means from those 

 to which c owos its existence. The valley formed by tho river 

 between the hills u and c would be styled a valley of denudation. 

 When tho subterranean force acts upon strata which has not 

 room to obey its impulse, the strata become crumpled OH c;i>ily 

 as the loaves of a book borne down by a weight upon their 

 edges. This phenomenon is illustrated in Fig. 3, whore the 

 sedimentary strata abut upon tho granite mass A, at u ; tho 

 contortions are numerous, but many instances no less remark- 



Fig. 3. 



able are known ; one is well exposed on the Bhine, where 

 tho Drachenfels wall in the river. At c, the whole series of 

 strata has bent in accordance with tho pressure, forming a 

 saddle-back or anticline (anti, opposite, and clino, I bend), be- 

 cause the strata dip in opposite directions from the summit of 

 the hill on whoso crest runs the anticlinal line. The Swiss 

 Jura may be quoted as a specimen of the formation sketched 

 at c. These mountains are composed of three such anticlines 

 running in parallel ridges. The valleys between B and c and 

 c and D would be called valleys of elevation. At D the strata 

 would be said to outcrop. If the outcrop be a bluff, bold cliff, 

 it is termed an escarpment. Tho angle which the direction of 

 the strata makes with the horizontal line is called the dip. 

 The line of the outcrop is termed the strike, that is, the direction 

 of the face of the hill as seen from the plain E. It is necessary 

 also to determine not only the angle of the dip, but the direction 

 of the dip, which will evidently be at right angles to tho strike ; 

 that is, if the strike run north and south, then the direction of 

 the dip will be east and west. 



When strata lie evenly upon each other and parallel, they arc 

 said to be conformable ; but when such an instance as E occurs, 

 where the overlying strata are not parallel to those beneath, but 

 have been deposited after the disturbed strata had assumed 

 their new position, they are then said to be unconformable. 



It may easily be imagined that sometimes the strata is too 

 brittle to permit of a bend such as c, it then breaks on the 

 crest of the ridge; this cleft becomes water- worn, forming a 

 valley running along the top of the mountain. An example of 

 this is also furnished in the Jura range. 



The modes in which the unstratified rocks are found asso- 

 ciated with those of aqueous origin are either in disrupting, 

 intersiratified, or overlying masses. 



In Fig. 4 the strata has been opened at c, and the fissure 



Pig. 4. 



filled from below with molten rock. This waa much harder 

 than the stratified rocks around it, whicli succumbed to the 

 action of wator, and became degraded, leaving the mass of hard 

 igneous rock standing out of the ground like a wall ; hence such 

 phenomena are termed dykes. At E is figured a specimen of 



ttratybd igneous rock. The molten matter WM ejected 

 over the floor of the ocean at a time when sediment WM in th 

 course of deposition, strata accumulated orer thin mac*, and 

 then another eruption broke through E and ite overlying strata, 

 pouring out another layer of nnstratined rock; when the whole 

 was raised from the ocean bed and submitted to tho denuding 

 action of water, tho hard bod H preserved the softer under* 

 lying strata; tho water having bat little action on B, left it 

 exposed, running along the side of the hill an a terrace. It often 

 happens that several of these igneous layers than appear one above 

 the other, and for thw reason hare 

 acquired tho name of trap rocks (from 

 the Swedish word trapp, a stair). 



At v in shown a smaller flavor*, 

 whose ramifications para through 

 stratified and unstratified rocks. 

 Such fissures are usually filled with 

 mineral ores ; they then are called 

 lodes (from the Saxon I'ul, a course) 

 and veins. 



The irruption of igneous rock* 

 necessarily produces dislocation of 



Fig. 5. 



the strata through which they pass ; when tho angle of the dip 

 alters, as at A, a fault is the result. But if the continuity of 

 the strata be broken, as at B, then a slip is said to have been 

 caused. These faults and slips are of vital importance to the 

 miner, as sometimes the seam of coal or the metalliferous vein 

 has slipped practically beyond his reach. 



The facts which have now been described are applied to 

 draw geological sections of countries and localities Li the follow- 

 ing manner. It is impossible to trace a bed of rock through 

 the whole of its development ; we meet with it here and there 

 at the sea-shore in a railway cutting in a quarry, or in 

 some ravine where a stream of water has worn its way; at 

 these different points we take its bearings, and then complete 

 the section from our knowledge of the general flexures which 

 strata undergo. 



The observer must carry with him a compass and a clinometer. 

 This latter instrument may be a quadrant of cardboard, the edge 

 of which is divided into 90 degrees. A piece of slight wood is 

 fastened with a pin to the angle of the cardboard, which is placed 

 in the direction of the dip of tho strata by the eye ; a plumb-line, 

 also attached to the pin, will show when tho upper edge of the 

 clinometer is horizontal. The angle between this upper edge 

 and the rod is tho dip of the strata. A clinometer is given in 

 Fig. 5. They may be bought with a compass attached to them 

 and a spirit level, so made as to fold together into a small space. 



Thus provided, the expedition may be commenced. The notes 

 of the observations are registered, as in Fig. 6. The observer is 



Fig. 6. 



guided by his compass to walk along the same direction, say 

 from south to north. Ho finds tho strata at A dipping southwards 

 at an angle of 25 ; a few miles further, after passing the crest 

 of a slight eminence, the strata is again exposed at B in a quarry 

 and here the dip is changed to northwards and tho angle to 

 30. Again at c an observation is made, where the dip is 

 southwards, and the angle 20. Hence between B and c the 

 strata evidently form a basin, tho steeper side being at B. At 

 D the dip is again northwards at a small angle, when suddenly 

 at E the angle is greatly increased, while the direction of the 

 dip remains the same. This could only be accounted for by 

 supposing that somewhere between D and E the strata had been 

 dislocated, and a fault produced. 



From these observations the section represented may bo 

 drawn. Tho three parallel lines at A, B, c, etc., indicate the 

 direction of the strike. 



