Ch. XV.] ON THE DISPLACEMENTS OF STRATA. 333 



CHAPTER XV. 



DO THE PRIMARY ROCKS AFFORD PHYSICAL EVIDENCE THAT 

 THEY HAVE EXPERIENCED FISSURES, DISLOCATIONS, AND 

 OTHER MECHANICAL MOVEMENTS ? 



The apparent indications of displacements. The curvatures of rocks. Alter- 

 nations of curved and straight strata. Veins often tortuous in straight beds. 



The colouring ingredients of rocks arranged in undulating and contorted 

 lines. Similar appearances in agates and alabaster also in igneous rocks. 



Sir James Hall's explanation of curved strata. The moving power 

 imaginary. The spheroidal structure of rocks. Primary conglomerates 

 not fragmentary the secondary, sometimes mechanical, at others concre- 

 tional. Igneous and aqueous rocks also conglomerated. Granite- veins 

 their structure and composition incompatible with theory analogous to 

 slate-veins and to certain arrangements in crystalline rocks. Mineral 

 veins different kinds of veins of segregation in Cornwall. Definition 

 of true veins objected to. Intersections of veins no criterion of their 

 relative ages. Remarks on the heaves and other supposed movements of 

 veins. Conclusion. 



THE subject of this chapter has been in part anticipated in 

 the discussion on the supposed elevation of the primary 

 strata ; and, certainly, on the determination of that question 

 the nature of some of the phenomena about to be considered 

 in a great measure depends; for these are generally pre- 

 sumed to be lesser movements, the concomitants of those 

 vast catastrophes by which the stratified deposits of all ages 

 were upraised from a horizontal position. 



The effects, however, of these supposed subordinate move- 

 ments present such varied appearances, and require to be 

 combated by such different arguments, that their individual 

 consideration cannot fail to elicit some instruction ; and, 

 indeed, unless they are passed under review, both by the 

 supporters and the opponents of the prevailing theory, the 

 discussion will not be satisfactory. 



