358 riiOF. J. W. JTJDD ON THE PllOPYLlTES 



Without laying any great stress upon the value of the distinction, 

 it may be convenient to group the propylites of the Western Isles, 

 of Scotland in two series. In one of these, the prevailing ferro-mag- 

 nesian silicate was originally hornblende or biotite, and this we may 

 speak of as the " Amphibolic Series." In the other group a pyro- 

 xene (augite or enstatite) was the predominating ferro-magnesian 

 silicate in the original rock, and this may be called the " Pyroxenic 

 Series." 



The rocks of the Amphibolic Series include most of the types 

 already spoken of as being distinguished by a paler colour, a lower 

 specific gravity (ranging from 2'-i to 2' 7), and a higher silica-per- 

 centage. The rocks of the Pyroxenic Series are usually darker- 

 coloured, have a higher density (2"6-2-9) and a lower percentage of 

 silica. But many varieties occur in which both pyroxenes and 

 amphiboles or mica are present, and in the case of the very highly 

 altered forms it is difficult and sometimes impossible to refer the 

 rock to either of these series. 



In both of these series we find rocks of highly crystalline cha- 

 racter (true diorites) passing through various hypocrystalline (" pilo- 

 taxitic " and " hyalopilitic ") varieties, into perfectly vitreous rocks. 

 Both the amphibolic and pyroxenic rocks sometimes contain free 

 quartz, and then pass into quartz-andesites and quartz-diorites. 



The chief types of the andesites and their Plutonic representatives 

 in the Western Isles of Scotland may be conveniently grouped as 

 follows : — 



A, Ampiiibole- and Mica-andesites. 



Hornblende-andesites. 

 Hornblende-mica-andesites. 

 Hornblende-mica-andesites with enstatite. 



Diorites and Quartz -dioi-ites. 



B. Pyroxene-andesites. 



1. " Vitrophyric " Pyroxene-andesites. 



2. '* Tracbytoid " Pyroxene-andesites. 



a. Stikkisholmur Type. 



b. " Diallage-andesites." 



c. Labradorite-andesites. 



Pyroxene-diorites and Quartz-pyroxene diorites. 



A. Amjohihole- and Mica-andesites and Diorites. 



The hornblende- and mica-andesites are perhaps more numerous 

 in the Western Isles of Scotland than the pyroxene-andesites. 

 While the former most commonly exist as quellekuppen and lacco- 

 lites, the latter more frequently constitute lava-schists and intrusive 

 sheets (" sills " of the miners of the Xorth of England) ; but this 

 distinction is far from being an absolute one, there being not a few 

 lava-streams, some broad and far-spreading, composed of hornbleude- 

 and mica-andesites, at the base of, or intercalated among, the j)latcau- 



