222 Dr. C. Callaicay — Bionte and Acidic Gneiss. 



into microscopic lenticles, saturated with mineral solutions, and 

 raised sometimes to the point of fusion. If a diorite can be traced 

 witliout a break into a reconstructed acidic rock, in which the 

 proportions of alumina and magnesia have been reduced, I conceive 

 that we are justified in concluding that, under enormous pressures 

 and at high temperatures, certain chemical changes are brought 

 abouJ; which are not produced under ordinary conditions. I do 

 not think I can fairly be called upon to demonstrate the exact 

 mode of operation. It is a step in advance to prove that the thing 

 has been done ; how it has been done is a further question. 



I may point out that there are at Malvern two principal types 

 of gradation between diorite and an acidic gneiss. In the Bagged 

 Stone Hill type — (1) the rock has been crushed into a laminated 

 mylonite previous to refusion and reconstruction ; (2) the shear- 

 zone contains no granite- veins, though a few felsite- veins are 

 present; and (3) magnesia, but not alumina, has been eliminated, 

 so that the resulting gneiss is highly micaceous. In the Swingard's 

 Hill type — (1) the rock becomes progressively sheared towards the 

 fusion-band, and mylonite is not produced; (2) the diorite is inter- 

 laced with countless granite-veins ; and (3) both magnesia and 

 alumina being eliminated,' the result is a quartzose gneiss or 

 gneissoid quartzite. 



Tlie diorite of the first type is a granitoid variety (my No. 3) ; 

 in the second series, it is medium-grained (my No. 1). The 

 chemical composition of both varieties is substantially the same. 

 The difference of behaviour under crushing may have something 

 to do with the greater or less compactness of the rock, the more 

 compact kind more readily slicing into lenticles while shearing. 

 Why alumina is sifted out in the one case and not in the other, 

 there is not sufiScient evidence to determine. 



I have been quite alive to the alternative view that the diminution 

 in the percentage of alumina may be due, not to the going out of 

 alumina, but to the coming in of silica. If this were admitted, it 

 would not affect the main contention of this paper — that, as a matter 

 of fact, the diorite does pass into an acidic gneiss. Furthermore, it 

 would confirm another of my conclusions, that, in the Malvern 

 metamorphism, chemical energy has been exceptionally intense ; 

 for this required silica can only have been supplied by the adjacent 

 plutonic rocks. 



But the microscopic evidence points to the actual removal of 

 the alumina. If silica had been introduced, it would appear as 

 an intruder amongst crystals of felspar, mica, etc. ; but the slides 

 show nothing of the kind. The quartz appears as blebs and 

 granules within the contours of the felspar-crystals. Or it entirely 

 takes the place of the felspar, while the general shape of the crystal 

 is retained, and frequently traces yet remain of a micaceous sheath 

 to the crystals. Or several of the felspars are replaced by silica, 

 and yet scraps of the biotite survive amidst the quartz to outline 

 the position of former felspar-crystals. 



1 In addition, of course, to other bases. 



