ox THE TROPYLITES OF THE WESTERN ISLMS OE SCOTLAND. )j41 



20. The Peoptlites of the Western Isles of Scotland, and their 

 Eelation to the Andesites and Diorites of the District. Ey 

 Prof. John W. Judd, F.R.S., F.G.S. (Read Pebruary 5, 1890.) 



[PlATES XIV. & XV.] 



I. Introduction. 

 II, Previous Literature. 

 III. Physical Characters and Chemical Composition of the Scottisli Pro- 

 pyl ites. 

 IV. Microscopical Characters. 

 V. Eelations of the Scottish Propylites to the other Eocks of the District. 



A. Geological Age. 



B. Striic'ture. 



C. Nahore of Eock-masses. 



1. Lava-streams. 



2. "Cupolas." 



3. " Laccolites." 



VI. Nature of the Original Eocks from which the Propylites have been formed 



A. Amiohibole- and Mica-andesitcs and Diorites. 



B. Pyroxeiic-andedtes and Pyro.voie-diorites. 



VII. Causes by which the Propylitic Modification of these Eocks has been 

 brought about. 



A. Solfataric Action. 



B. Contad-MetaiHorphisni. 



VIII. Light thrown by the Study of these Q>rtiai-y Lavas on some of the Older 

 Volcanic Eocks (Porphyrites, Felstones, &c.). 

 IX. The Younger Augite-andesites ("Tholeites," " Pitchstones," &c.) of the 



Western Isles of Scotland. 

 X. Summary of Eesults. 



I. Introditotion. 



There exists in the AVestern Isles of Scotland a great series of lavas 

 which, for the most part, underlie the ophitic oliviue-basalts, and 

 constitute the oldest of the ejections of the great Tertiary volcanoes 

 of that district. These rocks were distinguished by me in 1874 

 under the old English field-name of " Pelstones," and it was stated 

 that they " vary in colour from black, through various shades of 

 green and grey to white ; but in almost all cases their surfaces 

 acquire a white crust in consequence of weathering action "*. 



In attempting, at that date, to define more exactly the characters 

 of these lavas, by studying them microscopically in thin sections, T 

 was confronted by two difficulties. In the first place, it soon be- 

 came manifest that these more acid lavas of the Western Isles in- 

 clude a great variety of types — difi'cring Avidely from one another in 

 mineralogical constitution and in structure ; and in the second 

 place it was found that the minerals of which these rocks were 

 built up were in a remarkably altered condition. 



In both these respects, the " felstones " present a very striking 

 contrast to the overlying series of ophitic olivine-basalts. The latter, 

 as I have shown f? are remarkable for their uniformity of composition 



^ Q. J.G. S. vol. XXX. (1874) p. 236. 

 t Ibid. vol. xlii. (188G) p. 49. 

 Q.J.G.S., No. 183. 2 c 



