SoLLAS — On a Fragment of Garnet Hornfels. 49 



The presence of so large a proportion of manganese is a 

 feature of some interest, and brings to mind the " coticule " 

 described by Professor Renard, in which this constituent is pre- 

 sent in even greater quantity, as will be seen from the analyses 

 given in columns II. and III. after Drs. Mark and Pufahl.^ In 

 our rock a part of the manganese present in the coticule appears to 

 be represented by iron oxide. With a view to determining the dis- 

 tribution of this constituent, the mineral components of the rock 

 were separated by means of heavy fluids : they fell out in the 

 following order : — First, ilmenite and garnets (the latter of spe- 

 cific gravity 4'lt)), then manganese mica (sp. gr. from 3"01 to 

 2-795), next muscovite, and finally quartz and felspar. The resi- 

 due of quartz and felspar was unfortunately not preserved, owing 

 to an impression that the latter mineral was not present. This 

 is the more to be regretted, as it is possible that an analysis of 

 the felspar would have presented features of unusual interest. 



Garnets. — These yielded on analysis the results given in 

 column I. ; in column II. these are reduced to molecular pro- 

 portions, and in column III. is given the theoretical composition 

 per cent, deduced from the formula adopted below : — 



100-64 



The following is the formula which most nearly agrees with 

 the results of analysis : — 



4 (3 SiOs, AlA, 3 MnO) 

 4 (3 SiO^, AlA, 3 FeO) 

 3 SiOj, Fe^Oa, 3 MgO 

 3 Si02, Fe^Oa, 3 CaO 



The difference between the amount of ferrous and ferric oxide 

 found and calculated may be due to errors of experiment; the 



^ * Renard : Recherches sur le Composition et la Structure des Phyllades Ardermais, 

 p. 38, 1882, sep. copy.' 



SCIEN PROC. K.D.S. — VOL. VII., PAKT I. E 



