On the Genesis of Some Scottish Minerals. 217 



To the former category must be referred such changes as 

 the conversion of Amphibole into Biotite by dynamic causes, 

 of which very fine examples occur in many parts of Scotland ; 

 and also the better known case of the conversion by dynamic 

 causes of potash felspars into Muscovite. Many Biotite 

 schists and Muscovite schists have arisen in this way, 

 mainly, of course, in the more ancient rocks. The genesis of 

 both Muscovite and Biotite offers a most interesting and 

 instructive example of the fact that almost any mineral may 

 originate in two or more ways. There can be no doubt that 

 Biotite (using this name in its widest sense) does occur as 

 an original constituent of some eruptive rocks ; it is also 

 one of the most common products of contact-metamorphism ; 

 and there can be no doubt that Biotite is due in other cases 

 to the paramorphism of Hornblende, which mineral itself 

 may once have been one of the Pyroxenes. Nay, further, 

 Biotite may arise in connection with the minerals coming 

 under the next category. The case of Muscovite presents 

 an almost parallel instance. It is undoubtedly an original 

 constituent of some highly silicated eruptive rocks, though 

 it is not of common occurrence in that connection in Scotland. 

 It is certainly developed in a few cases by thermo-meta- 

 morphism. As a product of the dynamic metamorphism of 

 arkoses and other clastic rocks of felspathose composition, 

 such as the Torridonian arkoses, as well as of eruptive rocks 

 containing potash felspars, it is one of the commonest and 

 most widely diffused of Scottish minerals. Furthermore, like 

 Biotite, it certainly does arise in connection with the minerals 

 whose origin falls next to be described. We may take these 

 as typical examples of minerals whose origin is due to 

 dynamic metamorphism. 



When w^e come to consider the origin of the various 

 minerals which are due in some way to the effects of dynamic 

 metamorphism operating concurrently with the introduction 

 of constituents from an outside source, we find ourselves 

 face to face with some of the most difficult problems in the 

 whole field of geognosy. It has long seemed to me that the 

 whole question of the origin of these various minerals re- 



