352 PROF. J. AV. JUDD ON THE PEOPYLITES 



tite, much of which is of secondary origin, has been converted into 

 various forms of hydrous brown oxide, and not unfrequently we 

 find great quantities of pyrite and marcasite developed, doubtless at 

 the expense of the magnetite and ilmenite. In the thin white crust 

 that so constantly covers these rocks the iron has been reduced and, 

 in some cases, removed in solution by the action of water containing 

 organic matter, as already pointed out. In some cases this removal 

 of iron-oxides has gone on throughout the whole substance of the 

 rocks, which become completely bleached. 



While in most cases the alteration of the ground-mass and the 

 conversion of the porphyritic constituents of the rock into pseudo- 

 morphs has not wholly destroyed its original aspect, yet, under 

 certain circumstances, as I shall point out, the whole structure and 

 character of the mass is found to be completely transformed. This 

 is effected by the crystallizing-out of different minerals (among 

 which the epidotes and chlorites are the most conspicuous) at the 

 expense of the various secondary minerals that have been deve- 

 loped in the mass by the alteration of the felspars and ferro- 

 magnesian silicates. These excessively altered varieties frequently 

 constitute rocks of very great beauty and interest*. 



From this general account of the chemical, macroscopical, and 

 microscopical characters of these Scottish rocks every one familiar 

 with the accurate descriptions given by von Eichthofen, Zirkel, 

 Doelter, Becker, and other petrographers of the "greenstone-tra- 

 chytes " of Hungary and Transjivania, and of the " propylites " of 

 California, Nevada, and Utah, will at once perceive their complete 

 identityf. 



[Since this paper was read I have had an opportunity of showing 

 Mr. J. P. Iddings, of the U.S. Geological Survey, a series of speci- 

 mens and sections of the Scottish propylites. He was able to 

 satisfy himself of the close similarity between these rocks and those 

 of the Washoe district in Kevada, which he has so carefully studied ; 

 and he has permitted me to state his conviction of their identity in 

 character.] 



I shall show that, as in the districts mentioned, the curiously 

 modified propylites of the Western Isles of Scotland have been 

 produced from dacites and andesites, and from the deeper-seated 

 and more highly crystalline representatives of those rocks, by the 

 operation of certain well-defined agencies. 



* Dr. Hatch, to whom a number of sections cut from these rocks were 

 submitted by Dr. A. Geikie, lias i'ully recognized the completely altered cha- 

 racter of the materials he examined. Unfortunately he had no uieans for 

 •judging of the real nature of tne rocks from which these were derived (Trans. 

 Roy. Soc. Edinh. vol. xxxv. 1888, pp. 77, 167). 



i" See the excellent summary of these results given by Rosenbuseh, ' Massige 

 Gesteine ' (1887), pp. 690-693. 



