56 



OUTCEOP OF STEATA. 



[Ch. V. 



tions, as expressed by the arrows. The emergence of strata at the sur- 

 face is called by miners their outcrop or basset. 



If, instead of being folded into parallel ridges, the beds form a boss 

 or dome-shaped protuberance, and if "we suppose the summit of the 

 dome carried off, the ground plan would exhibit the edges of the strata 

 forming a succession of circles, or ellipses, round a common centre. 

 These circles are the lines of strike, and the dip being always at right 

 angles is inclined in the course of the circuit to every point of the com- 

 pass, constituting what is termed a qua-quaversal dip — that is, turning 

 each way. 



There are endless variations in the figures described by the basset- 

 edges of the strata, according to the different inclination of the beds, 

 and the mode in which they happen to have been denuded. One 

 of the simplest rules with w^hich every geologist should be acquainted, 

 relates to the V-like form of the beds as they crop out in an ordinary 

 valley. First, if the strata be horizontal, the V-likc form will be 

 also on a level, and the newest strata will appear at the greatest 

 heights. 



Secondly, if the beds be inclined and intersected by a valley sloping 

 in the same direction, and the dip of the beds be less steep than the 

 slope of the valley, then the Vs, as they ai-e often termed by miners, 

 will point upwards (see fig. 74), those formed by the newer beds appear- 

 ing in a superior position, 

 and extending highest up 

 the valley, as A is seen 

 above B. 



Thirdly, if the dip of the 

 beds be steeper than the 

 slope of the valley, then 

 the Vs will point down- 

 wards (see fig. '75), and 

 those formed of the older 

 beds will now appear up- 

 permost, as B appears above 

 A. 



Fourthly, in every case 

 where the strata dip in a 

 contrary direction to the 

 slope of the valley, what- 

 ever be the angle of incli- 

 nation, the newer beds will 

 appear the highest, as in 

 the first and second eases. 

 This is shown by the draw- 

 ing (fig. 76), which exhib- 

 its strata rising at an angle 

 siopeofTaiiuy2uo, dip of strata 50°. of 20°, and crossed by 



Eig. T4. 



Slope of valley 40°, dip of strata 20°. 

 Fig. 75. 



