234 On Mineral Veins. 
rider a appears to have been separated from the wall of the 
vein by the gradual filling and expansion of a joint at 0. 
At ¢ dis shown a similar joint filled with ore, where, if the 
deposition of minerals continued, the vein would form along 
the joint ¢ d, and the enclosed block would be converted 
into a rider. Sometimes these riders remain 7m situ, in 
other cases they have been lowered for some distance, pro- 
bably through the decomposition and removal of the consti- 
tuents of the vein on which they had rested. The riders in 
flat veins present much the same appearance as those in the 
vertical veins, and have evidently originated in the same 
way. 
To recapitulate. We have opposed to the theory that 
mineral veins have been open fissures caused by some dis- 
ruptive force, either acting at once or by a succession of 
movements, and filled by the gradual deposit of minerals 
brought from below in currents of water circulating in the 
open fissures. 
1st. The impossibility of any fissure remaining open, even 
to the smallest extent, in most of the rocks traversed by 
vertical or horizontal veins. 
2nd. The fact that this theory will not account for the 
filling of the wedge-shaped veins, nor for the formation of 
the surface bunches and the detached leads or strings - of 
quartz. 
3rd. The phenomena attending the occurrence of riders 
both in flat and vertical veins. 
In support of this theory instances have been adduced, 
where the deposit of minerals assumes the form of corres- — 
ponding parallel layers on each side of a vertical central rib. 
But it should be noted that this is not the ordinary arrange- 
ment of the minerals in the vein—in fact, it is an appearance 
rarely met with, and then only in hard rocks, and extending 
over limited portions of the vein. The ordinary flat veins 
in limestone rocks may in some instances have been open 
cavities, or it would be more correct to say, that open cavities 
may have existed in some portions of these veins previous 
to the deposition of the ores ; and similar small cavities may __ 
occur in vertical veins traversing hard rocks, but these are 
exceptional cases, and their occurrence on this small scale 
does not remove the impossibility of whole mineral veins 
being formed in this wa 
On the other hand, nearly all the facts observed point to 
