164 Professor Keilhau on Contact Products. 



wacke, there rises the mural ridge of Cerrig-Mwyn, a mass of 

 grey quartz-rock, which is sometimes brecciated. In contact 

 with this projecting mass, which, in so far as the strike at 

 least is concerned, is parallel to the bounding slates, the latter 

 are much indurated, and contain very considerable portions 

 of lead-glance. This ore is likewise accumulated in large 

 quantities, just at the side of the quartz-rock. (Murchison's 

 Silurian System, vol. i. p. 366). 



Regarding all these facts, I must first be allowed to offer 

 some incidental observations. 



The whole of the examples now adduced, exhibit masses of 

 ore as contact-formations. Although it appears that it is 

 really metallic minerals which most frequently present them- 

 selves in this manner, still, the one-sided tendency of geology 

 must still so far bear the blame, that, up to the present time, it 

 is difl&cult to find in descriptions, other substances than ores 

 mentioned, as belonging to junctions of rocks whose non- vol- 

 canic origin cannot easily be called in question. It is only 

 when one of the rocks in contact is regarded as pyrogenic, 

 that the theorizing geologists trouble themselves with notic- 

 ing the peculiar mineral products occurring between them ; 

 and, on this account, we are acquainted with innumerable 

 examples of such cases, in masses which meet the crystalline 

 siliceous rocks. We are indebted almost exclusively to those 

 occupied with the practical department of the science for a 

 number of instances of a different description ; but, from this 

 cause, attention has naturally been chiefly directed to the ores, 

 and not to the other substances belonging to this group of 

 mineral products. 



With respect to the example indicated by the letter d, it 

 must be remarked, that when the repository there noticed is 

 designated a vein, perhaps no error has been committed. It 

 is, however, certain, that very many contact-repositories have 

 been improperly termed veins ; for where one of the two rocks 

 in juxtaposition is unstratified, the contact-masses are, for 

 the most part, quite irregular and disposed in lumps, and al- 

 together do not run so uniformly as true veins, which lie be- 

 tween the sides of a rent or fissure.* 



* I have formerly expressed the opinion, that to refer by far the greater number 



