AVATSON — NOTES ON METALLIFEEOUS SADDLES. 



359 



being sometimes maintained over a wliole country, althong-li generally 

 confined to a comparatively small portion of the strata. It happens 

 that the contortions, which are not to be confounded with those 

 larger effects of the same kind known as synclinal and anticlinal 

 axes, are so comphcated that, as in some of the oldest rocks of ISTorth 

 Wales, the folds are partly turned over, and the order of the strata 

 is actually inverted ; but such extreme cases are comparatively rare. 

 The country where the beds are thus disturbed almost invariably 

 displays other proofs of mechanical alteration, and usually more or 

 less of elevation. As conducting to the explanation of the dis- 

 turbances which may have taken place in a district these phenomena 

 are often very useful guides, since they nearly always indicate the 

 direction in which the disturbing force was applied, and this I shall 

 presently show. 



It may be demonstrated experimentally that plicated or folded 

 strata are the result of great lateral pressure, aided by much super- 

 incumbent weight. Sir James Hall — whose important experiments 

 in uniting chemistry with geology laid out a path which, unfor- 

 tunately for the progress of the latter science, in at least one depart- 

 ment, has been since too little followed — succeeded admirably in re- 

 producing the appearances in the rocks by placing plates of moistened 

 clay one over the other, with a heavy weight on the top of them, and 

 then squeezing them at the sides. The effect produced is re- 

 presented in the diagram (fig. 1), which will also serve as an illus- 

 tration of the contortions on the large scale, as in nature. The 

 appearances, however, are best imitated by thick paper, or cloth, 

 moistened by gum, or other adhesive liquid, which will cause the 

 sheets to retain the form they may assume, after the pressure is 

 withdrawn. It will be observed that a series of consinuous waved 

 lines are produced ; in fact, a miniature succession of anticlinals and 

 synclinals, and it is one of each, taken separately, that the miner calls 

 a saddle (see fig. 2). The crown may be either an unbroken arch 

 (ft) (fig. 2), or, if the squeezing and bending has been more severe 

 than the rock could stand without fracture, it may be an angle (c), 

 more or less acute. From the crown downwards there is usually a 

 well marked fissure, or joint, traversing all the beds in succession 

 with more or less inclination from the vertical (fig. 2, B B'). The 

 sides of a saddle are termed its wings, h V (fig. 2), and the crown is 

 called the huckle (ct) ; the joint dividing the crown is called the 

 saddle-joint ( ft B, c B'). The space between two saddles at its lower 

 part, which on the large scale would be termed a synclinal axis, is 

 called the trough (c) (fig. 2), and, as it is usually fractured like the 

 crown, the dividing fissure is called the trough joint, indicated by 

 the line d c 



What I have just described relates more particularly to the lime- 

 stone saddles than to the plicated beds of the calcareous and bitu- 

 minous shales which overlie the limestone, since in the latter, 

 although the same general structure prevails, there is more confusion 

 in the strata. 



