158 Professor Keilhau on Contact Products, 



isolated occurrence in extremely rare small portions, in the 

 midst of enormous masses of crystalline silicide rocks, as a 

 very remarkable circumstance. The rare substances which 

 they contain, were not, as is remarked by Mr Scheerer, so dis- 

 tributed as silicium, calcium, potassium, or sodium, for in that 

 case they must have been more frequently met with. If it be 

 now asked, whether it is to be assumed that, in such a case, these 

 materials were not originally present at the places where the 

 minerals containing them now make their appearance ? — I main- 

 tain, that, in consequence of the phenomena exhibited by con- 

 tact-formations, the answer may very well be in the affirmative. 

 We still know nothing as to whether they were there produced 

 at a later period " by means of an inexplicable chemical pro- 

 cess," or whether we must suppose that they were conveyed 

 thither in some way or other from other places ; let, however, 

 the study of such invariably parasitic formations be but once 

 undertaken, so that the relations of some of them may throw 

 light on those of the others, and we may then hope to obtain 

 some insight into this subject likewise. 



Another case, in which the phenomena of contact-forma- 

 tions may become explanatory, is that of the origin of veins, 

 and especially of metallic veins. In another publication I have 

 already expressed myself on this subject, in the following 

 terms {Einiges gegen den Vulkanismus, p. 78) : as shifts almost 

 always occur near metallic and mineral veins, and as they 

 generally bring into contact the transverse terminations of 

 halves of beds not belonging to one another, we easily perceive 

 the analogy between the occurrence of such veins in stratified 

 rocks dislocated by fissures, and those mineral products which 

 we have specially denominated contact-formations. The ideas 

 arising from the study of these last mentioned products, be- 

 come, however, by continued attentive consideration, still more 

 comprehensive as regards the theory of veins. When two 

 heterogeneous masses touch each other'J forces are brought 

 into operation, by means of which chemical products are called 

 forth ; but is it not also probable, that in the formation of a 

 fissure, the tearing asunder of one and the same rock disturbs 

 the tranquillity of these forces, and brings them into action % 

 Certainly one disturbance or the other of the previously exist- 



