150 Professor Keilhau on Contact Products. 



The information to be derived from contact indurations is 

 very important ; and what they tell us of their own origin 

 has also its application to the question of the formation of 

 certain mountain rocks. Clay-slate, where it is in contact 

 with granite, is frequently of the nature of hornstone ; but 

 the change likewise proceeds much farther. The hornstone- 

 like condition is but the beginning of a series of modifications, 

 which at last exhibits to us gneiss-formations as a product of 

 conversion ; sandstone strata, which, at some parts of their 

 junctions with other mountain rocks, have only become harder 

 and more homogeneous, are, at other points of the same 

 junctions, converted into mica-slate or crystalline quartzite ; 

 in short, both these investigations regarding indurations and 

 silicifications have to deal, not only with the production of 

 these themselves, but also with the mode of formation of a 

 whole class of important rocks. And yet, with how little 

 attention geologists have gone to work in the examination of 

 these phenomena ! After it had been found that the indura- 

 tions in some degree resemble imperfectly melted masses, and 

 with the possession of the undeniable fact, that melted masses 

 of great volume must act with a fusing effect on the sand- 

 stones, slates, &c. with which they come in contact in the hot 

 state, it was without hesitation assumed as a general principle, 

 that all the indurated slates occurring at junctions are more 

 or less perfect products of this description. Here the irra- 

 tionality of the method of investigation is very evident. Not 

 only, in deciding as to whether a rock is of eruptive and pyro- 

 genic origin, have the contact-changes occurring near it been 

 employed as an infallible criterion, but, as already hinted, the 

 important question, which must necessarily arise in a cautious 

 and logical investigation, has been entirely neglected or sup- 

 pressed, — whether, namely, the phenomenon is really exclu- 

 sively connected with such rocks as may be supposed to have 

 been at one time in a hot state l and, in so far as it occurs 

 near masses which have actually at one time possessed a 

 high temperature, whether it was not produced after these 

 were cooled \^ As to the chemical part of the question, but 



* If we turn our attention to the subject, we may convince ourselves that con- 

 tact-actions still go on. In the coal-field of Northumberland, hydrogen gas con- 



