90 J. M. Clements — Study of Contact Metamorphism. 



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and 2. It will be seen that there is only about -J-th as much 

 potash in the contact rocks as in the normal clay slate ; while y 

 on the contrary, about 12 times as much soda as there was in 

 the slate has been added to the contact rock. This causes a 

 reversal of the relations of the soda and potash, so that, whereas 

 in the clay-slate there is present 10 times as much potash as 

 soda, we find in the contact rock taken as an example very 

 nearly 10 times as much soda as potash. 



Comparison of Mlneralogical Composition. 



It will be sufficient for our present purpose to consider the 

 clay-slate, on the one hand, and the spilosite and adinole on 

 the other. To recapitulate briefly, in the clay-slate we have 

 quartz (clastic), white mica, actinolite, rutile, and an indeter- 

 minate interstitial material of finely granular character, discol- 

 ored by carbonaceous and ferruginous matter. This intersti- 

 tial material we may consider as that remnant of the original 

 feldspar, quartz, biotite, and iron ore dust in the original clay 

 which was not used in the production of the white mica, actino- 

 lite, and rutile now found in the clay-slate. Original chemical 

 differences in the slate are shown by the arrangement of the 

 various materials in bands, certain bands containing a greater 

 quantity of white mica ; hence, the conclusion that these were 

 probably richer in alkali than the adjoining bands, which are 

 poor in white mica. 



In the spilosite and adinole we find present quartz — but not 

 clastic — white mica, actinolite, and rutile, as in the clay-slate. 

 But in addition there is albite, chlorite, epidote, and biotite. 

 There is nothing in these rocks which shows clastic origin. 

 The original sediment has been completely recrystallized and 

 there remains only the banding to point to the sedimentary 

 origin of the rock. The clastic quartz has been destroyed by 

 solution or aqueous fusion, and has been recrystallized, and 

 now forms a mosaic with the albite. Albite, for whose pro- 

 duction some of the silica of the clastic quartz may have been 

 used, is present in the rock in considerable quantity, in contra- 

 distinction to its total absence from the slate. The initial 

 quantitative chemical differences existing in the clastic rock are 

 also shown in the recrystallized rock by differences in the min- 

 eralogical composition, a large quantity of chlorite, for instance, 

 forming a band adjacent to a band poor in chlorite, thus indi- 

 cating a richness of the first band in magnesium. The great- 

 est difference existing between the practically un metamor- 

 phosed slate and the contact product is the evidence in this last 

 of complete recrystallization, and the presence of albite in large 

 quantity. 



