Professor Keilhau on Contact Products. 149 



or less the form of beds, notwithstanding that, in other respects, 

 all the circumstances seem to be the same as those in which 

 the silicification of the slate so frequently presents itself. 2. 

 The very extensive silicifications of the clay-slate, occurring 

 in the same district in the vicinity of the great masses of 

 granite and syenite, cease where the boundary between the 

 slate and the granite or syenite passes near the underlying 

 gneiss. There, instead of the usual horn-slate, we regularly 

 find perfectly soft alum-slates, sometimes containing embedded 

 needles of chiastolite. Supposing that the gneiss has there 

 really performed an active part, this may, perhaps, be in some 

 respect compared to the operation of catalytic bodies. 3. 

 Where, in the same district, whole zones of several thousand 

 feet in breadth, round the granite, consist of altered strata, it 

 very often happens that, near the boundary of these zones 

 which is away from the granite, some completely unaltered 

 strata are to be observed between others which are altered in 

 the usual manner. This occurs in the most remarkable manner 

 at a place where the strata do not strike towards the granite, 

 but past it. There we find that, where the altered zone passes 

 into the unaltered slate-formation, there is a regular alterna- 

 tion of modified and unmodified strata, so that several of the in- 

 durated strata are entirely separated from the granite by unin- 

 durated strata* (Gaea Norvegica, i. p. 16.) 



The necessity is evident of observing such relations, and 

 not passing them over in silence, whether they lead to this or 

 that result. That hitherto similar phenomena have been little 

 or not at all noticed in other places, plainly arises from the 

 mode in which geology is at present prosecuted. If the fa^ 

 vourite theory were placed aside for a time, and if more truly 

 philosophical principles were adopted, a multitude of hitherto 

 unnoticed facts would be discovered, and new light would be 

 thrown on many obscure subjects. 



* G. Rose, in his ^eise nach dem Ural, gives a description and representation 

 of a cliflf near Orsk, which, in its upper portion, consists of hypers thene-rock, under 

 which lie jasper and clay-slate. Of the last, it is said that it contains many beds 

 of flinty slate. As Rose regards the jasper as an altered clay-slate, and as the 

 flinty slate can only be a transmuted clay-slate also, and that merely in an in- 

 ferior degree to the jasper, we have here, undoubtedly, a phenomenon of the same 

 kind as that occurring at the junction of the granite near Christiania. 



