Dr. C. Callaway — Chlorite and Biotite. 219 



succession in time on the same spot. A mass of diorite begins to 

 suffer from a crushing process, aqueous solutions, in which both 

 carbonic acid and potash occur,' soak into it, and chemical changes 

 take place. The pressure increases, shearing supervenes, tlie tem- 

 perature rises, and certain other chemical reactions result. There is 

 no rushing about from one place to another. All the materials for 

 the genesis of the biotite are already present, before the higher 

 temperatures set in. 



To be more precise I will briefly state some of the actual changes, 

 as observed in the field and the microscope, and confirmed by 

 chemical analysis. Crystals of hornblende and felspar are lying 

 side by side. Some of the latter contain potash, perhaps as the 

 result of contact action. Crushing sets in, and foreign solutions 

 enter the rock. The crystals of felspar are broken to fragments, 

 and some of it is decomposed, alkalies and lime being liberated. 

 The hornblende also is crushed and decomposed, chlorite, iron-oxide, 

 and calcite being produced. The materials for the production of the 

 biotite are present, but increased heat and pressure are necessary to 

 excite the required chemical affinities. In an advanced stage of the 

 metamorphism, the water of the chlorite, and some of its magnesia, 

 are driven out, and their place is taken by iron-oxide and potash. 

 That, at high temperatures, iron has a strong affinity for silica, is 

 proved by the well-known fact that, in the consolidation of igneous 

 magmas, iron-silicates come very early in the order of crystalliza- 

 tion. According to Lagorio they come next to oxides and before 

 any other silicates. 



I do not think that General McMahon will attach much import- 

 ance to his objection that, if the temperature of the rock "rose to 

 the point of fusion," the chlorite must have been melted down and 

 merged in the general magma, and, therefore, could not have given 

 rise to biotite. It is sufficient to remind him that the fusing-point of 

 chlorite is much higher than that of felspar, and that, in granite, 

 quartz usually consolidates after felspar. It would, therefore, be 

 exceedingly rash to assume that a temperature which would fuse 

 quartz and felspar,* which form the bulk of the rock, would also 

 melt the chlorite. My assertion that, in scores of sections, chlorite 

 is seen to be replaced by biotite as we approach the granite-diorite 

 shear-zones, will hardly be invalidated by such a speculative objection 

 as this. 



My critic denies that the conversion of chlorite to biotite is " an 

 observed fact." But, surely, if Lessen, Michel Levy, Salomon, and 

 Eudermann, not to mention my humble self, have observed the 

 change, it may fairly be regarded as proved. Besides, I thought 

 that General McMahon himself admitted that the thing might be 

 true as the result of " contact action." I confess I do not quite 

 understand his position. 



1 Not necessarily contemporaneously as potassium carbonate. 



* Professor Redway, of Mount Vernon, New York, commenting on my Malvern 

 papers (" Science," February 9th, 1894), describes the fusion in the shear-zones as 

 " sufficiently complete to produce plasticity." 



