TRANSACTIONS OF SECTION C. 593 



proof. When sections of felspar, such as occur in thin slices of igneous rock, are 

 examined under the microscope in polarised light, petrologists can distinguish one 

 species from the other — when the direction in which the sections were cut is 

 approximately known — by measuring the angles at which they extinguish from 

 the twinning or the pinacoidal plane. 



This is not mere theory. Each species of felspar has its own angle of 

 extinction and its own index of refraction. The determination of these two 

 factors enables a petrologist to prove optically the change in composition; or, in 

 other words, the change in species which has taken place in the successive 

 zones, during the gradual growth of a large zonal felspar. 



Another general rule must now be mentioned. I think it may safely be 

 asserted as a broad rule, that the different species of felspars are attackable by 

 the chemical reagents which make themselves felt in metamorphie action, in the 

 order of their basicity ; that is to say, the more basic felspars are more easily 

 attacked than the acid ones. When we bear in mind the facts stated above, 

 we shall, I think, be able to see clearly how it is that the peripheral portions 

 of large felspars in igneous rocks sometimes escape alteration, whilst the cores 

 of these crystals are converted into secondary minerals, such as chlorite, silvery 

 mica, zoisite, epidote, kaolin, steatite, saussurite, calcite, and scapolite. 



The chemical reagents flowing in solution through the pores of the felspars, 

 pass by the more acid and refractory species, and devote their energies to the 

 more susceptible basic species entombed at the heart of the zonal crystals. 



The point I wish to enforce most strongly is that the phenomenon above 

 described, namely, the formation of secondary metamorphie minerals in the 

 interior of a crystal, combined with the comparative immunity to change of the 

 external portions, shows that the agents which brought about chemical changes 

 at the core of the crj'stal flowed freely through its unaltered peripheral portions. 



But some may ask whether the chemical agents referred to may not have 

 gained access to the heart of a crystal by a crack. I answer that a crack is a 

 coarse and tangible object that looms large under the microscope. A crack in a 

 mineral liable to metamorphie action, through which chemical reagents have 

 flowed, could not escape detection. The finest crack through a homogeneous 

 mineral, such as, for instance, an olivine, can be readily seen, not only by the 

 small canal worn by the corrosive action of the chemical agents that flowed 

 throTigh it, but by the alteration set up in the mineral along the whole course of 

 the canal. 



I have a thin slice from a beautifully fresh olivine contained in one of the lavas 

 of Vesuvius collected by myself. A volcanic explosion or other cause, operating 

 after the crystallisation of the olivine, produced a very fine crack in the mineral 

 through which water, charged with chemical reagents, subsequently flowed. The 

 crack, though of microscopic width, is filled with serpentine, and on both 

 margins fibrous serpentine has been formed at the expense of the parent olivine, 

 and constitutes a fibrous band on both sides of the crack throughout its entire 

 length, the direction of the fibres being at right angles to the crack. 



The rest of the olivine is of virgin purity and polarises in the most brilliant 

 colours, contrasting strongly with the serpentine. 



In this case it is clear that the chemical reagents, though free to flow along 

 the crack, had commenced to extend beyond its walls, encouraged thereto by the 

 porosity of the olivine itself. But how difl'erent is this case from those in which 

 the entrance of the chemical agents had not been facilitated by a crack. In the 

 case above described, the chemical changes set up were limited to the borders of 

 the crack, and even had they gradually extended in the course of time to the 

 whole of the olivine, the original canal by which the chemical reagents had 

 gained access to the crystal would have remained to tell its tale, and exhibit 

 along its course the banks of iron oxide thrown down by the chemical navvies 

 that had excavated it. 



Cracks save time as roads and canals do, but they leave behind them evidence 

 of their former existence. In order to understand fully how rocks and minerals 

 are so completely open to the attacks of chemical reagents, which penetrate to and 



1902. Q Q 



