464 Notices oj Memoirs — 



chlorite, silvery mica, zoisite, epidote, kaolin, steatite, saussurite, 

 oalcite, 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 metamorphic 

 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 crystal 

 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 meta- 

 morphic 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 through 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 

 different 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 re- 

 mained to tell its tale, and exhibit along its course the banks of iron 

 oxide thrown up 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 produce chemical and 

 mineralogical changes at the very hearts of minerals, we must fully 

 realise how completely porous rocks and minerals are to the heated 



