1922.] ROCKS OF ALDERNEY. 143 



face. Material which might be muscovite, but has higher 

 refractive index, slightly more colour, and oblique extinc- 

 tion, suggests tremolite. Here, as with the general mass of 

 material, there is development of epidote. One section ex- 

 hibits a large fractured fragment of black iron oxide with 

 epidote formed between the severed portions, There is a 

 small quantity of crushed quartz, and reticulated needles of 

 unrecognised material, perhaps sillimanite, in a clear mineral, 

 and in other parts of the slide. 



Irregular parallel lines of haematitic iron ore bordering 

 epidote granules, suggest the invasion of water along nar- 

 row cracks in the rock. Field evidence is ample to indicate 

 movement and shear on a large scale. 



DOLERITE DYKE. The rock has reached a stage of de- 

 composition which obscures the original pyroxene or amphi- 

 bole. There is now a fair quantity of apparently secondary 

 felspar surrounded by black magnetite patches, largely 

 shapeless. Many felspar particles have an octagonal outline 

 bordered by faint brown indeterminate material, and mottled 

 with iron ore. Larger examples of colourless plagioclase 

 occur, with repeated twinning and fairly low refractive 

 index. The whole texture is even grained, and there seems 

 no evidence of ophitic structure. In fact the felspar some- 

 times includes corroded rounded bodies of presumed augite, 

 now pale brown, with no cleavage cracks and slight inter- 

 ference colours. Some of the small pale-brown patches 

 appear to have been an amphibole, and some chloritisation 

 has occurred. In places, water infiltration has deposited 

 strings of brown material. Except for magnetite, nothing is 

 distinctive in ordinary light, and in polarised lieht onlv the 

 felspars are determinable. Mr. A. K. Wells, M.Sc, F.G.S., 

 has suggested that it is a beerbachite. 



" MlCA-TRAP " DYKE. The principal feature is a fine dis- 

 play of straw-coloured biotite, with normal characteristics, 

 occupying almost half the entire area. The ground mass of 

 much finer material contains a large amount of the same 

 mineral, with smaller, pale-coloured laths of plagioclase of 

 low extinction angle in reference to the vertical axis. The 

 remainder of the rock is indistinct, being made up of an 

 aggregation of haematite, with magnetite and brown material 

 in formless confusion. 



In regard to the direction of the magnetite veins, there 

 appears to have been augite, but no augite remains now. 

 Many magnetite veins, traversing brown semi-opaque ma- 

 terial, suggest an amphibole origin. The rock seems to be- 

 long to the kersantite type of the lamprophyres. 



