72 W. M. Hutchings — Clays, Slates, and Contact-Metamorphism. 



re-formation of the felspar which originally formed part of the 

 deposits of the slates, but which no longer existed in them as such. 



(4) White mica. — In soine, probably in most, contact-areas a good 

 deal of white mica is newly-formed. It may not be always easy to 

 trace the exact stages of the process. The original slates are usually 

 rich in white mica to begin with, and the recrystallization of the 

 mineral in the earlier stages of contact-metamorphism is not readily 

 distinguishable. But in the more advanced stages we see it more 

 plainly, and find a good deal of mica which from its general appear- 

 ance and relationship to the other minerals is clearly re-formed, this 

 being also demonstrated frequently, as to larger individuals, by their 

 being more or less charged with quartz-grains, etc., in the same 

 manner as are the biotite-flakes and crystals, and the other newly- 

 formed minerals. 



In addition to this mica which appears plainly to have re- 

 crystallized in a manner exactly similar to that of the quartz and 

 felspar, we often see also the mica of which I have spoken as 

 appearing by various stages in the newly-formed substance to which 

 60 much reference has been made, and which in the fullest form of 

 its development will remain as patches among the mosaic, or as 

 fringes round its component minerals, and may not in all specimens 

 be visibly connected with any residual portions of the substance 

 out of which it originated. 



(5) Cordierite, Andalusite, Cyanite, SiUimanite. — These are the 

 minerals we may perhaps designate as more specially " contact- 

 minerals," and we may safely say of them that the conditions of 

 their formation, and why sometimes one is formed and sometimes 

 another, are the points as to which we probably linow least of all. 



As regards cordierite, it appears to stand in a somewhat diiferent 

 position from the rest. It is not a silicate of alumina only, and its 

 formation appears to take place often at a much earlier stage of the 

 metamorphism. It is also a much less stable mineral than the 

 others. 



Looking at the composition of cordierite and the mode of its 

 occurrence in these contact-rocks, the most reasonable supposition 

 as to its origin appears to be that when the recrystallizations and 

 rearrangements of the minerals commence, the cordierite, when 

 formed, largely represents the magnesia which is combined in the 

 mica of the slates, — mica which we call sericite and which we have 

 good reasons to regard as muscovite into whose composition magnesia 

 has been taken. As recrystallization of this mica takes place, it 

 will have a tendency to purify itself, and to approach more nearly 

 to the normal muscovite, with more potash and less of other bases ; 

 and the magnesia will be available for the formation, more usually 

 of biotite, but under conditions we cannot specify, more or less of 

 cordierite forms also.' 



1 It seems desirable to point out that there is risk of sometimes confusing cordierite 

 and felspar in such rocks, where the cordierite occurs as smaller grains and is not a 

 striking constituent. In the books we are warned against mistaking cordierite and 

 quartz; but a similar warning does not seem to be given as regards felspar. As 

 regards quartz the risk does not appear to be great. Where there are several grains 



