i 



Professor Keilhau on Contact Products, 159 



ing equilibrium follows the discontinuity that has been pro- 

 duced, and, at all events, two masses now exist where there 

 was only one formerly. It will be observed, that I am now 

 pointing out how, in general, metallic and mineral veins may be 

 brought under the same category with contact-formations. 



I have already remarked, that it cannot really contribute 

 much to simplify the chemical explanation of the occurrence 

 of mineral products developed at junctions, if we assume that 

 one or other of the rocks in contact has had an extraordinarily 

 high temperature. In relation to the subject, let us consider 

 the following appearance described by Leonhard. Near Auer- 

 bach, there is, in the gneiss, a " vein" of granular limestone, 

 which, near the walls, is full of idocrase. Leonhard is of opin- 

 ion, that, when the limestone, in a hot liquid condition, came 

 in contact with the gneiss, separations and combinations of the 

 elementary constituents of the two rocks took place, and that 

 in this manner the idocrase was formed. If the principle were 

 fully admitted, that liquidity must be supposed when such 

 operations as this take place, then, with reference to the che- 

 mical explanation of the phenomenon, a necessary reason 

 would appear for bringing forward the hypothesis of the for- 

 mer liquid condition of the limestone. But, at present, the 

 object of this hypothesis can only be to open up the path for 

 farther explanation. Does it actually accomplish this? I doubt 

 it much. It would be interesting to hear what the chemists 

 themselves would say on the subject. But let us suppose that 

 this phenomenon, chemically considered, is really thus rendered 

 more intelligible, and, at the same time, let us see how this 

 advantage is obtained. It is only by doing violence to all geo- 

 logical probability that we can assert that the limestone has had 

 the origin ascribed to it by the hypothesis ; for the eruptions 

 which are known to have taken place have never exhibited any 

 thing of the kind, whereas, on the contrary, w^e meet with crys- 

 talline limestone containing minerals composed of silicates, 

 which, we know with certainty, was never in a melted state (see 

 first part of this paper, vol. xxxvi. p. 357). But, moreover, this 

 hypothesis can only be brought forward at the expense of the 

 eruption doctrine itself. So long as this doctrine endeavours 

 to explain phenomena by the general state of liquidity of the 

 interior of the earth, and, when the question is as to the source 



