Professor Keilhau on Contact Products, 151 



little regard has been paid to those strict principles which 

 have been laid down respecting the mutual relation of geology 

 and chemistry ; for, in so far as the altered deposits have be- 

 come siliceous or richer in silica than they previously were, 

 this is a matter in which chemistry is still pretty much in 

 arrear. 



When an uncrystalline slate, like that already noticed, has 

 been converted into gneiss, for example, at a junction with 

 granite, individual crystalline portions of felspar, mica, and 

 quartz, have been formed at the boundary ; and these minerals 

 are properly distinguished as actual contact-products, in con- 

 tradistinction to those results of juxtaposition which consist 

 of more or less considerable modifications of previously exist- 

 ing masses. There is a great number of these contact-pro- 

 ducts ; as well of those which occur as disseminated mine- 

 rals, as of such (particularly ores) as constitute very exten- 

 sive masses of different mineral species. Among many other 

 topics connected with these remarkable products, which I 

 cannot now discuss, are the following : — Their subdivision, 

 according to a more exact determination of their situation, 

 in so far as they either occur in one or other of the rocks 

 which are in contact, at a little distance from the junction, or 

 actually between them ; and their subdivision into those in re- 

 gard to which it may be assumed that the material existed in 

 the immediate vicinity of the places where they are now found, 

 and those in whose case such a supposition appears to be in- 

 admissible. At present, I shall only treat of these products, 

 in so far as is incumbent on me, in order still farther to make 

 good the assertion, that the present practice in geology is essen- 

 tially deficient, inasmuch as nothing is listened to regarding 

 contact-phenomena, except in connection with volcanism, and 

 then, of course, only in its favour ; the actual contact-products 

 are perseveringly and obstinately regarded as the mere result 



tinues to be evolved at the side of a vein of basalt. (Boue'a Jahretbericht for 1822, 

 p. 12). In the Magasin for Naturvidenskabeme, vol. ix. p. 72, attention is 

 directed to a fact which probably bears on the same subject. It is there stated, 

 in a series of observations to determine the intensity of terrestrial magnetism in 

 a portion of Central Europe, that there is an extremely remarkable magnetic rela- 

 tion in the valley of Fassa, precisely where the well-known granite of Predazzo 

 comes in contact with the limestone. 



