144 ROCKS OF ALDERNEY. 



Granite-Porphyry. The rock shows a fine ground-mass 

 with porphyritic crystals. The phenocrysts are of quartz and 

 felspar. The quartz is well formed, clear, with inclusions 

 and uncorroded. The felspar is alkali felspar, the ortho- 

 clase showing Carlsbad twinning. There is also microcline 

 and an alkali-plagioclase with very low angle of extinction 

 and showing traces, dying out in the section, of albite twin- 

 ning. There is corrosion of felspar phenocrysts by the 

 ground-mass, the effect showing irregularities in the traces 

 of the crystal face. Phenocrysts show considerable kaolini- 

 sation. 



There is a small quantity of scattered iron ore, mainly 

 black but some brown-coloured. The ground-mass shows 

 an intergrowth of quartz and felspar, and is micropeg- 

 matitic. The quartz is clear, the felspar slightly turbid. 

 There are signs of movement, the fracturing of quartz show- 

 ing disturbance after consolidation. The rock is a typical- 

 granite porphyry. 



SECTIONS ILLUSTRATING THE GEOLOGY OF 

 ALDERNEY. 



1. Repeated faulting in the Conglomerate trending mainly 



N.N.E.-S.S.W. It appears to be an earlier phase 

 than the major Armorican movement. 



2. The pre-Cambrian block faulting to westward. East- 



ward, the Cambrian deposit on the dioritic sea-floor. 



