Ob. V.] 



ANTICLINAL AND SYNCLINAL LINES. 



57 



Slope of valley 20°, dip of strata 20°, In opposite 

 directions. 



a valley, which declines in an 

 opposite direction at 20°.* 



These rules may often be of 

 great practical utility; for 

 the different degrees of dip 

 occurring in the two cases 

 represented in figures 74 and 

 75, may occasionally be en- 

 countered in following the 

 same line of flexure at points 

 a few miles distant from 

 each other. A miner un- 

 acquainted with the rule, 

 who had first explored the valley (fig. 74), may have sunk a vertical 

 shaft below the coal-seam A, until he reached the inferior bed B. He 

 might then pass to the valley fig. 75, and discovering there also the out- 

 crop of two coal-seams, might begin his workings in the uppermost in the 

 expectation of coming down to the other bed A, which would be observed 

 cropping out lower down the valley. But a glance at the section will 

 demonstrate the futility of such hopes. 



In the majority of cases, an anticlinal axis forms a ridge, and a syn- 

 clinal axis a valley, as in A, B, fig. 62, p. 48 ; but there are exceptions 

 to this rule, the beds sometimes sloping in- 

 wards from either side of a mountain, as in 

 fig. 77. 



On following one of the anticlinal ridges 

 of the Jura, before mentioned, A, B, C, fig. 

 71, we often discover longitudinal cracks and 

 sometimes large fissures along the line where 

 the flexure was greatest. Some of these, as above stated, have been en- 

 larged by denudation into valleys of considerable width, as at C, fig. 71, 

 which follow the line of strike, and which we may suppose to have been 

 hollowed out at the time when these rocks were still beneath the level of' 

 the sea, or perhaps at the period of their gradual emergence from be- 

 neath the waters. The existence of such cracks at the point of the 

 sharpest bending of solid strata of limestone is precisely what we should 

 have expected ; but the occasional want of all similar signs of fracture, 

 even where the strain has been greatest, as at a, fig. 71, is not always 

 easy to explain. We must imagine that many strata of limestone, chert, 

 and other rocks which are now brittle, were pliant when bent into their ' 

 present position. They may have owed their flexibility in part to the 



Fig. T7. 



* I am indebted to the kindness of T. Sopwith, Esq., for three models which I 

 have copied in the above diagrams ; but the beginner may find it by no means 

 easy to understand such copies, although, if he "were to examine and handle the 

 originals, turning them about in different ways, he would at once comprehend 

 their meaning, as well as the import of others far more complicated, which the 

 same engineer has constructed to illustrate faults. 



