368 PROP. J. W. JUDD ON THE PEOPYLITES 



from carefully selected specimens, and taken from different portions 

 of a prop3iite-niass, are examined microscopically. 



We are able to see how clear plagioclase with vitreous lustre 

 (microtine) becomes gradually clouded and opaque, and in the end 

 completely kaolinized ; the changes being, in the first instance, 

 developed along the planes of chemical weakness between the twin 

 lamellae ; but the extension of this action very frequently results in 

 the complete obliteration of all traces of the original twin-lamellation. 

 In some instances the felspar substance then breaks up into a 

 mosaic of different minerals, among which zoisite and a secondary 

 felspar usually appear to play the most important part. But in other, 

 and perhaps the majority of instances, the results are modified by the 

 impregnation of the products of the felspar alteration with ferro- 

 magnesian secretions derived from the decomposition of other 

 minerals in the rock. The consequence of this is that the necessary 

 materials for the formation of an epidote are brought together and 

 tufted masses of pistacite, or some other variety of that species, are 

 formed and replace a part or the whole of the felspar crystal. 



The ferro-magnesian silicates at the same time lose their dis- 

 tinctive character, and green isotropic products (viridite) are formed 

 at their expense. Out of these decomposition-products various 

 chlorites are formed with the separation of secondary magnetite. 

 These, in turn, yield to furtlier chemical action, and pistacite and 

 other epidotes are produced, forming more or less distinct pseudo- 

 morphs after the pyroxene, amphibole, or mica *. 



The ground-mass is often one of the earliest portions of the rock 

 acted upon. Any glassy matter that may be present disappears as 

 the result of secondary devitrification, and the whole matrix of the 

 rock is frequently converted into a mass of secondarj' minerals. 

 Among these, various metallic sulphides are often very conspicuous. 



When steam-holes abound in the rock the epidotes and other 

 secondary minerals crystallize out freely ; and in these situations 

 they are conveniently displayed for careful study and determination. 

 Beautiful amygdules, composed of epidote and other secondary 

 minerals, and filled in with still later deposits of zeolites, calcite, 

 and chalcedony, are, indeed, among the most conspicuous features 

 presented by the surfaces of such of these propylitic rocks as have 

 originally possessed a scoriaceous character. 



There are localities in which the kind of change which I have 

 been describing seems to have been carried to its farthest ex- 

 treme. In these cases ej^idote has been developed to such an extent, 

 at the expense of the other constituents, that it is now quite im- 

 possible to determine the original mineralogical constitution and 

 structure of the rock. The most marked example of this is seen in 

 the eastern spurs of the great mass of Beinn More in Mull, and 

 around ^A Chioch. Here the epidotization of the rock-constituents 

 and the formation of numerous volcanic minerals has gone on to 



* Compare Q. J. G. S. vol. xlii. (1886) pp. 430, 431. 



