Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 59 



two planes of less resistance meet, it is very possible that it may be 

 propagated smniltaneously along these planes. These diverging branches 

 may contmue separate, and present themselves at the surface as two 

 distmct fissures, or they may meet again, and thus including a portion 

 of the mass in which they are formed, produce tlie phenomenon above 

 mentioned. If the insulation be perfect, the mass, if not too laroe will 

 of course faU, and may descend to any unknown depth; and possibly 

 this may be one cause of the partial irregularities in the width of the 

 hssures of mineral veins. If the insulation be imperfect, or the width 

 of the mass be greater than that of the fissure immediately beneath it 

 It will be supported in its original position, or it may under other cir- 

 cumstances lodge at a certain depth below it. In either case if such 

 a mass come within the sphere of the miner's observation, he terms it 

 a rider, (Introd. ii. ^). 



If the rider be originally supported as above suggested, till a suf 

 ficient quantity of matter shall have been deposited in the fissure to 

 afford a support to it independent of its contact with the walls and 

 the fissure be then increased in width by any renewed action similar 

 to that which originally produced it, the rider may present itself to 

 us supjDorted by the vein-stuff, in a state of perfect insulation from 

 the solid mass on either side of the vein*. 



61. In the phenomena attending the intersections of veins, described 

 m the Introduction (ii. o, ,r, p,) the broken veins are generally supposed to 

 have been originally continuous, and to have been broken by a relative 

 movement of the portions of the mass on opposite sides of the unbroken 

 vein. Adopting this hypothesis, we have not the smallest difficulty 

 m accounting for the appearance represented in the figures, p. 6. (Introd 

 II. p,) since our elevatory force must necessarily produce in many cases 

 that relative elevation of different sides of a fissure, which at once ac 

 counts for the phenomena in question. The other two cases above 



- This perfect insulation of riders has been recently urged as an objection of the most 

 serious weight against the mechanical origin of veins. It appears to me, on the contrary to 

 be an almost necessary consequence of the causes we are considering actmg on a mass con 

 ■stituted like the crust of the earth. 



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