66 



LYELL'S ELEMENTS OF GEOLOGY. 



Curved Strata. 



nearly horizontal, are still brought up twice by a slight curva- 

 ture to the surface, once on each side of A. Beginning at the 

 north-west extremity, the tile-stones and conglomerates No. 4. 

 and No. 3. are vertical, and they generally form a ridge paral- 

 lel to the southern skirts of the Grampians. The superior strata 

 Nos. 2. and 1. become 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 per- 

 ceive by the. diagram, that in travelling 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 leaving the 

 newer, and advancing upon older strata. All the deposits which 

 he had before examined begin then to recur in reversed 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 to- 

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

 in the same relative order of superposition, but with a northerly 

 dip. At Whiteness (see diagram) it will be seen that the inclined 

 strata are covered by a newer deposit, a, in horizontal beds. 

 These are composed of red conglomerate and sand, and are 

 newer than any of the groups, 1, 2, 3, 4, before described, and 

 rest unconformahly upon strata of the sand-stone group, No. 2. 



Fig. 59. 



Curved strata of slate near St. AbVs Head, Berwickshire. 



An example of curved strata, in which the bends or convolu- 

 tions of the rock are sharper and far more numerous within an 



