Vol. 68; 



PETROLOGICAL NOTES ON GUERNSEY, ETC. 



41 



Pig. 2. — Gneiss-inclusion, por- 

 phyritic in the lower hut not 

 in the upjper portion. 



[A = Gneiss, felspars measuring not 

 more than half an inch ; maxi- 

 mum thickness, about 6 or 7 

 inches. 



B=Gneissoid, after the break granu- 

 lar rather than porpbyritic. 



C = Inclusion continued for about 

 2 feet apparently by a granular 

 streak. 



D = ' Clean ' band of ' red ' dyke. 



E, E = Granular streaks. 



F, F== Streaked red dyke-rock.] 



the whole of a dyke, exhibits the 

 slightly wavy sage-green streaks. 

 Here and there is a thin band of 

 uniformly red rock, which, in one 

 or two cases, seems as if some 

 clean magma had broken into the 

 ' dirtied ' one, while the latter 

 was solidifying. Sections have 

 been prepared to illustrate these 

 structures. One, in which are 

 not only green streaks but also the 

 above-mentioned granular bands, 

 is found to consist of (quartz, 

 and felspar in rather elongated, 

 irregular grains, and an altered 

 biotite (PI. I, fig. 3). The felspar 

 is rather decomposed, but Carls- 

 bad twins of orthoclase, microcline, 

 and a plagioclase, with rather 

 small extinction-angles, can be 

 recognized. The biotite is now 

 altered in colour to green, and 

 has extruded some iron-oxide in 

 the usual way : the flakes being 

 sometimes about -01 inch long, 

 but generally less (down to about 

 •002 inch). Where these are 

 more conspicuous, the felspar 

 distinctly exceeds the quartz in 

 quantity. The slice contains 

 also a little iron-oxide and apatite. 

 Another specimen, from a dyke 

 5 or 6 yards thick, with alternat- 

 ing streaks of red and greyish 

 green (the former dominating), 

 exhibits constituents generally- 

 similar in nature and shape; but 

 the altered biotite is less in 

 amount and smaller in size. A 

 third slice is from a dyke about 

 10 feet wide, rather compact in 

 structure, and in colour a fairly 

 bright red, with curving dull 

 green streaks, not exceeding 

 0*1 inch in thickness, recalling 

 the fluxion-structure of a rhyo- 

 lite. This consists of the above- 

 named minerals, but locally 

 shows a slight tendency to be- 

 come ' graphic' A fourth slice 



