HORNBLENDE-SCHISTS, GNEISSES, ETC. OF SARK. 



131 



with the rock described above, the only difference being that in the 

 present case the last-named mineral is associated with more quartz 

 and felspar, and in places is intercrystallized with a certain amount 

 of biotite. 



A third specimen of the gneiss, showing bands of pale reddish 

 (quartzo-felspathic) rock and of one more micaceous, differs from the 

 first in that some of the felspar grains are rather larger than the 

 rest, as in a slightly porphyritic rock, and that in the more mica- 

 ceous band a few grains of green hornblende occur. These are 

 very irregular in form, and are associated with flakes of brown 

 mica or some rather minute and fibrous hornblende. Their appear- 

 ance is suggestive of a partial destruction of a large crvstal 

 (fig. 3). 



Fig. 3. — Hornblende^ ajpparently partly replaced by and associated 

 with biotite. 



Magnified 50 diameters. 

 Hornblende dotted ; biotite faintly lined ; quartz and felspar left white. 



It may be well, before drawing an inference from this section, to 

 notice briefly the evidence to be found in Port a la Jument. There 

 many lenticular pieces of the usual hornblende-rock (generally 

 rather smaller in size than in the last case) occur in a banded 

 gneiss, consisting usually of light quartzo-felspathic and dark 

 micaceous layers of various thickness, as above described ; the mass 

 of hornblende-rock sometimes resembles torn-off slabs, sometimes 

 appears to have been drawn out like a viscid mass ; here and there 

 in the gneiss a thin band of hornblende can be traced for several 



