48 



CURVED STEATA. 



[Ch. V. 



dish shale, containing peculiar organic re- 

 mains. A glance at the section will show 

 that each of the formations 2, 3, 4, are re- 

 peated thrice at the surface, twice with a 

 southerly, and once with a northerly indi- 

 cation or dip, and the heds in No. 1, which 

 are nearly horizontal, are still brought up 

 twice by a slight curvature to the surface, 

 once on each side of A. Beginning at the 

 northwest extremity, the tile-stones and 

 conglomerates No. 4 and No. 3 are verti- 

 cal, and they generally form a ridge par- 

 allel to the southern skirts of the Grampi- 

 ans. The superior strata Nos. 2 and 1 be- 

 come less and less inclined on descending 

 to the valley of Strathmore, where the 

 strata, having a concave bend, are said by 

 geologists to lie in a " trough" or " basin." 

 Through the centre of this valley runs an 

 imaginary line A, called technically a 

 " synclinal line," where the beds, which 

 are tilted in opposite directions, may be 

 supposed to meet. It is most important 

 for the observer to mark such lines, for he 

 will perceive by the diagram, that in trav- 

 elling from the north to the centre of the 

 basin, he is always passing from older to 

 newer beds ; whereas, after crossing the 

 line A, and pursuing his course in the same 

 southerly direction, he is continually leav- 

 ing the newer, and advancing upon older 

 strata. All the deposits which he had be- 

 fore examined begin then to recur in re- 

 versed order, until he arrives' at the central 

 axis of the Sidlaw hills, where the strata 

 are seen to form an arch or saddle, having 

 an anticlinal line B, in the centre. On passing this line, and continuing 

 towards the S. E., the formations 4, 3, and 2, are again repeated, in the 

 same relative order of superposition, but with a southerly dip. At White- 

 ness (see diagram) it will be seen that the inclined strata are covered by 

 a newer deposit, «, in horizontal beds. These are composed of red conglom- 

 erate and sand, and are newer than any of the groups, 1, 2, 3, 4, before de- 

 scribed, and rest unconformably upon strata of the sandstone group, No. 2. 

 An example of curved strata, in which the bends or convolutions of 

 the rock are sharper and far more numerous within an equal space, has 

 been well described by Sir James Hall.* It occurs near St. Abb's Head, 

 * Edia Trans, vol. vii. pL 3. 



