1922.] ro6ks of ALi)feHNJfe^. 139 



APPENDIX. 



Petrological Notes on the Rocks of Aldekney. 



PALE GREY GRIT. Most of the fragments are of clear 

 quartz which shows signs of strain and fracture. The quartz 

 elements are notably angular; very few examples are well 

 rounded, and of these, not one is large. As a noteworthy 

 contrast, the felspar exhibits fairly well-rounded corners. 

 The strain on the felspars has produced considerable crack- 

 ing, frequently severing the specimen in two or three direc- 

 tions, one of which is often the bedding plane. No felspar 

 is fresh, and indications of twinning are rare. 



There is abundant evidence of the formation of sericite, 

 accompanying the kaolinisation of the felspar, and constitu- 

 ting much of the matrix, which is rarely distinctly quartzose, 

 but rather aluminous. 



Beyond muscovite and attenuated and twisted biotite, 

 other minerals are rare, although zircon can be found. Of 

 ferro-magnesian minerals, little more than haematic iron ore 

 now remains ; it accompanies biotite, and assists as a binding 

 material where found. 



PURPLE GRIT. The purple grit has a red-brown ferru- 

 ginous cement which occupies a much higher proportion of 

 the rock than the aluminous paste occupies in the pale grit. 

 The cement is sometimes seen in wavy and straggling wisps 

 and drawn-out masses seeming to thread its way unevenly 

 between the quartz particles. Aluminous paste of kaolinised 

 felspar is also present. The angular shape of the quartz 

 fragments is more marked than in the pale grey grit; the 

 individuals here being triangular, elongated, lenticular, and 

 acicular. The few small felspar specimens are kaolinised and 

 iron-stained almost beyond recognition. Muscovite is rare; 

 biotite is almost absent. Some of the felspathic ground 

 mass appears to be the remnant of completely decomposed 

 felspar. 



BlBETTE GRANITE. Quartz, occupying a large percentage 

 of the rock, was the last mineral to form, its boundaries being 

 defined in all cases by the adjacent minerals. It has been 

 considerably strained, and is plentifully traversed with 

 cracks, infilled with iron-stained secondary matter. Some 

 biotite is fairly preserved, the alteration process, when evi- 

 dent, being rather in the direction of iron ore segregation 



