﻿488 



aiE. J. A. THOMSON OX THE H.OKXBLEXDIC [Xov. 1908, 



that occurring in the peridotite, and proves incoutestably that the 

 brown hornblende is a xenocryst (fig. 7). 



Pig. 7. — The ^ mixed rocTc,'' Glendalouc/7i,s7ioiving xenocrysts ofhroun 

 Tiornhlende tuithin actinolite, sphene, iron-ores, felspar (dear), 

 mica, and chlorite (right-hand Iniver portion of the figure). 

 (Magnified 30 diameters.) 



[S=Sphene; A=Actinolib ; M=Mica ; C=:Cblorite.] 



A pale biotite, almost absent in some specimens, is extremely 

 abundant in others. It shows pleochroism from brown to colourless, 

 and a very low axial angle. Pleochroic halos are not so abundant 

 as in the actinolite, but seem to be determined by the same mineral. 

 The mica is never idiomorphic in habit, but consists of small 

 allotriomorphic plates apparently moulded on hornblende, apatite, 

 sphene, clinozoisite, quartz, felspar, and garnet. Its relation to 

 the hornblende, included within the largest plates, and always most 

 abundant around the actinolite- fans, bears out Prof. Sollas's sug- 

 gestion that it has been derived from this mineral by the action of 

 the intruding magma. A similar origin for the mica of hornblende- 

 rocks has been described by many authors. 



Although the mica is apparently moulded on quartz and felspar, 

 it may not be posterior to these minerals, but may have been itself 

 partly resorbed before their solidification. Skeletons of acicular 

 sphene occur within many of the micas and extend beyond their 

 margins, but are bounded beyond the mica-crystals by parallel 

 planes on which the felspars have been moulded (fig. 8, p. 489). 

 This suggests that the mica once had a greater extension. There 

 is, however, no border of iron-oxide to bear witness to a magmatic 

 resorption. 



