68 Diorites and Granites of Swift's Creek, 



finally by the increase of crystalline alteration in approach- 

 ing the intrusive masses, to have had a common origin and 

 to have "been all affected by the same influences. It cannot, 

 I think, be doubted that these influences were connected 

 with the intrusive masses, and entirely to be distinguished 

 from those other influences which produced the regional 

 metamorphism which has affected the Silurian formations 

 up to the very boundaries of the contact schists. 



The course which these contact alterations have followed 

 is, I think, indicated by the microscopic examination noted 

 in the description of diagram, section No. 2. 



The least-altered rocks which still clearly retain their 

 sedimentary character are seen to be highly charged with 

 chlorite, and to have been somewhat altered in structure by 

 the aggregation of crystalline quartz in lenticular masses 

 and narrow veins.* In passing across the strata towards 

 the contact it is found that the chlorite decreases pari 

 passu with an increase of mica, and thereby with an 

 increasing crystallinity of the rock until the immediate 

 contact schists (as at the Charlotte Spur) are found to con- 

 sist mainly of mica, with only faint traces of chlorite, or,. 

 as in other places where the contact rock is a typical horn- 

 fels, the chlorite has entirely vanished. These observations 

 I observe to be in accord with statements made by Pro- 

 fessor Rosenbusch as to the contact rocks Of Bar Andlau 

 in Alsace.-f These observations may be thus shortly sum- 

 marised : — 



It is evident that there is surrounding the great intrusive 

 masses a zone of altered sediments, but this zone is not a 

 zone of sediments enfolding the mass after the manner of an 

 envelope. The sediments are discordant in their bedding to 



* I cannot help suspecting that the colourless, doubly refracting plates, 

 which in many cases are seen to form the ground masses of such slightly 

 metamorphosed rocks as these, may be a mineral of the kaolin group in a 

 crystalline form. This view has been strengthened by the observation that 

 indurated, pale-coloured, upper Silurian mudstones in contact with an intru- 

 sive granitic mass at Kaffir's Hill, Foster, are seen under the microscope to 

 be entirely composed of minute plates, which are probably orthorhombic. 

 A qualitative examination of the rock showed it to be near kaolin in 

 character. 



f An inspection of the analyses of contact rocks from the normal Steiger 

 schiefer to the granitite contact, given by Professor Eosenbusch (Die 

 Steiger Schiefer, fye., Strasslurg, 1877, p. 256), will show that " with 

 increasing metamorphic intensity — with greater proximity to the granitite 

 the percentage of water in the contact products decreases in a regular and 

 visible manner." 



