DISLOCATION OF STRATA. 



99 



The following figures, from a paper by D. Sharpe, illustrate some of these distortions 

 accurring in a slate rock in Wales. They represent two species of shells, the Spirifer 

 disjunctus (Nos. 1 to 4) and the Spirifer giganteus (Nos. 5 to 8). No. 1 is the natural 

 form of S. disjunctus ; the others are distorted. The lines z z show the lines of cleavage 

 in the slate: 2 lay in the rock inclined 60° to the planes of cleavage, and is shortened 

 one-half: 3 lay obliquely at an angle of 10° or 15°; it is shortened above the middle 

 and lengthened below it: 4 is a cast, the upper part pressed beneath that shown, while 

 the lower is much drawn out: 5 is like 3, the angle with the cleavage-plane being less 

 than 5°; the lower part has lost its plications by the pressure and extension: 6 has a 

 similar angle to the cleavage-plane, but a different position : 7 intersects the cleavage- 

 plane at only 1°, and its lower part is very much prolonged. Compression, a sliding 

 of the rock at the cleavage-planes, and more especially a spreading of the rock itself 

 under the pressure, are the causes which have produced these distortions. All fossils 

 are liable to become similarly misshapen under the same conditions. 



Calculating the thickness of strata. — When strata are inclined, as 

 in Fig. 110, the thickness is ascertained by measuring the extent 

 along the surface, and also the angle of dip, and then calculating the 

 thickness by trigonometry. The thickness of the strata from a to b is 

 b d, the line b d being drawn at right angles to the strata. Measuring 

 a b, and the dip, which is the augle bad, the angles and hypothenuse 

 of the triangle a b d are given to determine one side b d. Or, with 

 the distance a e, the side c e would be found. 



But it is important, for trustworthy results, that the absence of 



faults be first ascertained. The figure (110) represents a fault at b g, 

 so that the strata 1, 2, 3, 4 to the left are repeated to the right ; and 



