8 Mb HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



V. Transverse Valleys. 



Deep valleys are sometimes found of which the directions are nearly 

 at right angles to that of the general elevation*. 



VI. Dykes or Veins, and Horizontal Beds of Trap. 



a. The dykes are usually found in nearly vertical planes, and, when 

 they occur in the vicinity of each other, with a general tendency to 

 parallelism. 



/3 Extensive beds of trap are found apparently interstratified with 

 the stratified rocks. 



VII. Granite Veins. 



The form of a vein of this kind is frequently very different from 

 that of mineral or trap-veins, as above described, inasmuch as a section 

 of it does not generally approximate in the same degree to rectilinearity f. 



These approximations to general laws have been, I believe, very 

 generally recognized by geologists, and more especially in faults and 

 mineral veins, in almost all cases in which these phenomena exist 

 throughout districts of considerahle extent; and this appears imquestion- 

 ably to justify the notion, that they are not to be referred to par- 

 tial causes, but to some cause general at least with reference to 

 the district throughout which the same laws are observed to hold with- 

 out breach of continuity. Local and accidental causes may in some 

 cases act with sufficient energy to obliterate all traces of general laws 

 in phenomena such as those above mentioned; but still this wiU mani- 

 festly not invalidate our inference with respect to those districts in 

 which such laws have been clearly recognized. We may moreover 



* These valleys may frequently be due in great measure to the effects of erosion. In some 

 instances, however, they appear to have been obviously formed by the elevation of the strata 

 on either side of them. The valley of the Wye, in Derbyshire, offers a beautiful example 

 of this kind of formation. 



t Trap veins sometimes assume the tortuous form of a granite vein. See M'CuUoch's 

 Description of the Western Islands of Scotland. Vol. III. PI. xxxiii. 



