Propylites of the Western Isles of Scotland. 287 



these rocks are to be found in the great volcanoes of Hawaii, and there 

 is nothing either in their fundamental characters or in their mode of 

 origin that cannot be paralleled among the products of causes now in 

 action. 



The authors expressed their indebtedness to Professors Bonney and 

 Judd, as well as to those who have preceded them upon the classic 

 ground of Mont-Genevre. 



2. " The Propylites of the Western Isles of Scotland, and their 

 Eelations to the Andesites and Diorites of the District." By Pro- 

 fessor John W. Judd, E.R.S., F.G.S., &c. 



The " Propylites " of von Bichthofen and Zirkel constitute what 

 has been aptly characterized by Eosenbusch as a " pathological 

 variety " of the andesites. The relations of rocks of this type to 

 the andesites and diorites in Eastern Europe and in the Western 

 Territories of the United States have been made known to us by the 

 researches of Dolter, Szabo, Becker, Hague and ladings, and other 

 petrographers. 



The " felstones " described by the author of the present memoir 

 as constituting the oldest series of the Tertiary volcanic rocks in 

 the Western Isles of Scotland are now shown to belong to this 

 interesting type. When found in an unaltered state, these rocks pre- 

 sent remarkable analogies with the andesites of Iceland and the Faroe 

 Islands, which have been so well described by Zirkel, Schirlitz, Osann, 

 and Breon. In the altered condition in which they usually occur, 

 however, the Scottish rocks resemble in a not less striking manner 

 the " propylites " of Eastern Europe and Western North-America. 



The rocks in question vary in colour from white to dark grey, various 

 shades of green usually prevailing among them. They have a specific 

 gravity ranging from 2*4 to 2*9 ; the density diminishing as the silica 

 percentage and the amount of glassy material in them increase ; a 

 lowering of the density of the rocks being also the result of extreme 

 alteration. In their chemical composition these rocks were shown to 

 agree with the pyroxene- and amphibole-andesites of other areas, 

 and with propylitic forms of those rocks. 



Yery striking and remarkable is the amount of change that many 

 of these Tertiary rocks have undergone — change that has equally 

 affected their porphyritic constituents and the ground-mass in 

 which these are imbedded. The felspars are never fresh, but are 

 more or less kaolinized, and not unfrequently converted into epidotes 

 and other secondary minerals ; the ferro-magnesian silicates are 

 almost always changed into isotropic "viridite," or into various 

 chlorites ; while the titano-ferrite and magnetite have been con- 

 verted either into " leucoxene " or into sulphides. The glass and 

 microlites of the original ground-mass have in nearly all cases dis- 

 appeared as the result of secondary devitrification. 



The propylites are the oldest of the Tertiary lavas in the Western 

 Isles of Scotland. They exhibit every gradation in minute struc- 



