Geological Society of London. 141 



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-Geuevre. 



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

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

 fessor John W. Judd, F.R.S., F.G.S., etc. 



The " Propylites " of von Kichthofen and Zirkel constitute what 

 has been aptly characterized by Rosenbusch 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 Iddings, 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 in- 

 teresting type. When found in an unaltered state, these rocks present 

 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. 



Very 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 l-esult 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- 

 ture, from holocrystalline forms (diorites) through various "grano- 

 phyric " and " pilotaxitic " types into true vitreous rocks ("pitch- 

 stones"). They are found constituting lava-streams, which are 

 usually short and bulky; eruptive bosses or " Quellkuppen " ; and 

 lenticular intrusions or " laccolites." 



