382 PKOF. J. W. JUDD ON THE PROP ELITES 



augite-andesites, the so-called " diallage-andesites " with augite 

 diorites and quartz-augite-diorites. 



The causes by which the " pro]Dylitic modification" of these 

 rocks has been brought about are two-fold ; namely, solfataric action, 

 which produces widely spread results, and contact-metamorpJdsm, 

 which is strictly local in its effects. By microscopic study of the 

 rocks, the actions produced by each of these causes can be 

 discriminated and severally studied. The solfataric action appears 

 to have accompanied the intrusion of the highly acid masses 

 (granites and felsites) of the district, and is shown to have taken 

 place at each of the five great volcanic centres previously described. 



The study of these greatly altered Tertiary rocks throws much 

 light upon the mode of origin of some of the most obscure among 

 the Palaeozoic lavas — rocks to which the names of " felstoue " and 

 *' porphyrite " have been applied. It is shown that while in some 

 cases these rocks are simply andesites which have undergone slight 

 alteration from the action of surface-waters, in other instances the 

 rocks in question must have been profoundly changed by solfataric 

 action and converted into propylites before the alteration from the 

 surface commenced. 



In striking contrast with the older Tertiary and much altered 

 andesites (propylites) of the district are the remarkablj^ fresh 

 volcanic rocks which are everywhere seen to intersect and overlie 

 the eroded masses of the plateau-basalts, and are therefore of much 

 later age than those rocks. These younger rocks which are only 

 preserved as surface lava-flows at the Sgurr of Eigg and at Ben 

 Iliant in, Ardnamurchan are of much interest, as constituting the 

 most recent volcanic rocks of the British Islands. They are shown to 

 have the most striking correspondence in their petrograj^hical cha- 

 racters with the rocks of the Tertiary dykes that traverse the south of 

 Scotland and the north of England, which have been described by 

 Er. A. Geikie, Mr. Teall, and other authors. These rocks, which were 

 in 1874 referred to the augite-andesites, are shown, both at Ben 

 Hiant and in some of the dykes, to illustrate in a remarkable way 

 the influence produced on the characters and chemical composition 

 of rocks when the same mineralogical constituents are united in 

 varying proportions. In this case we find every gradation from 

 highly basic holocrystalline rocks, through various " ophitic,'' " in- 

 tersertal/'' and " pilotaxitic " types of augite-andesite, into quite 

 acid " vitrophyric " andesites (pitchstone-porphyries). 



EXPLANATION OF PLATES XIY. & XV. 



I The system of notation here adopted to indicate tlie magnifying-power used 

 for the rock-sections is ex]jlain6d in the Quart. Journ. Geol. Soc. vol. xlii, 

 (1886), p. 88.] 



Plate XIV. 



In tliis Plate an attempt has been made to ilhistrate the chief characters of 

 the Older Tertiary Propylites, and of some of the Andesites, of which they are 

 the altered representatives. 



