58 Mh HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



fissures would necessarily diverge in all directions from the central 

 points, so that parallel systems such as above mentioned could not 

 possibly be thus produced. It has moreover been shewn, (Art. 30.) 

 that every system of parallel fissures in which no two consecutive 

 fissures are remote from each other, must necessarily have had one 

 simultaneous origin. Subsequent efforts of the subterranean forces may 

 enlarge these fissures, and propagate some of them to the surface, con- 

 verting incomplete into complete fissures, but it would seem essential, 

 according to our view of the subject, that their jMsifions in the lower 

 portion of the mass, where their formation will commence, (Art. 36.) 

 should be determined contemporaneously. 



§. Formatio7i of Eiders — Explanation of the Phenomena at the Intersections 



of Mineral Veins. 



59. If two systems of fissures were formed by forces acting in the 

 manner we have supposed on a mass without vertical or nearly vertical 

 planes of less resistance, these systems would present to us cases of 

 intersection only of nearly vertical fissures with horizontal beds, or 

 with other vertical fissures at right angles to the intersecting ones. 

 It is manifest, however, that the existence of planes of less resistance, 

 combined with an irregularity of intensity in the elevatory force such 

 as we have assumed, may produce some fissures irregular both in 

 direction and inclination to the horizon, though the general pheno- 

 mena may still present that distinct approximation to the laws we 

 have indicated, which would be the necessary consequence of the great 

 predominance of general over local causes. It is at the intersections 

 of the two perpendicular systems of veins (metalliferous veins and cross 

 courses) that the most important of the phenomena we are about to 

 consider are found, while others occur at the intersections of veins of 

 more irregular formation. 



60. Before we proceed to examine these phenomena more particu- 

 larly, we may notice one probable consequence of this occasional irregu- 

 larity in the formation of veins, viz., the production of what are usually 

 termed riders. If a fissure be propagated through a point in which 



