210 Derby — Notes on Certain Schists of the Gold and 



resulted from the decay of a rock very similar to the one 

 analyzed. 



Nos. V, YI and YII were analyzed by Prof. Gorceix for 

 the express purpose of determining whether the denomination 

 u chloritic" or " talcose," ordinarily applied to the predomi- 

 nant schists of the region, was applicable or not. They may, 

 therefore, be presumed to be the most typical unctuous schists 

 that could be found in a sufficiently sound state to be analyzed. 

 All three show a considerable amount of soda, which in No. Y 

 is in excess of the potash, so that the micaceous element can- 

 not be a normal muscovite. Alumina is somewhat in excess of 

 what is required to satisfy the alkalies as normal muscovite 

 and it was noted that macroscopic pyrophillite occurs in the 

 bed from which No. YI was taken, so that it is probable that 

 the excess of alumina may, in all three rocks, represent a small 

 percentage of this mineral, or of cyanite, which is also very 

 frequent throughout the region. Free iron oxide was noted in 

 No. YI in the form of altered pyrite and it is probable that 

 No. Y also carries a small amount in the same state, or in that 

 of hematite. All three show a considerable excess of silica, 

 doubtless in the form of quartz, and the small percentage of 

 magnesia probably represents a slight admixture of chlorite. 

 Nothing is known of the heavy residues of these rocks or of 

 the details of their mode of occurrence. 



No. YIII has almost exactly the amount of silica and 

 alumina required to satisfy the alkalies and magnesia as normal 

 muscovite and chlorite, leaving an excess of about 8 per cent 

 of silica for quartz. As in Nos. II and IY the iron is prob- 

 ably, for the most part, in the state of hematite. 



All of these rocks show a low to extremely low percentage 

 of silica and a high to extremely high one of alumina, while 

 all without exception are poor in lime and, except No. I, in 

 magnesia as well. On the other hand, all but No. 1 show 

 tolerably high percentages of alkalies. 



From these analyses no certain conclusion as to the original 

 mineral ogical composition and mode of origin of these rocks 

 can be drawn. On the somewhat risky assumption that the 

 present chemical composition of metamorphic rocks is, as 

 regards non-volatile constituents, essentially identical with the 

 original composition, no satisfactory comparison with known 

 types can be made for the majority of these rocks. Nos. Y 

 and YII agree fairly well in composition with some of the 

 trachytic and phonolitic tuffs cited in Zirkel's Petrographie,. 

 vol. iii, pp. 675 and 680. If, as is unusually done with such 

 rocks, they be considered as metamorphosed arkose, the material 

 must have been derived from a syenitic (or porphyritic) type 

 rich in soda rather than from an ordinary granite or gneiss. 



