142 HOCKS OF ALDEKN&Y, 



derived hornblende, the boundaries of one mineral merging 

 into the other. The rock seems originally to have been a 

 gabbro or quartz-gabbro. 



The Alderney granites and diorites are apparently 

 hybrid rocks, formed by interaction of differentiated phases. 



Gneiss, near Dolerite Dyke. No constituents have 

 well-defined outlines. The margins of the various mineral 

 sections are blotched and indistinct. The general alteration 

 effect either thermal, or dynamic, or both, is a confusion of 

 material, the inclusion of ferro-magnesian matter in felspar, 

 and the invasion of felspathic matter into chloritie debris. 



Few minerals are in evidence. Quartz, wedged between 

 felspar and chlorite in small granules, is rare. It appears to 

 have escaped fusion or semi-fusion. Felspar is predominantly 

 plagioclase, apparently oligoclase-albite by its low refractive 

 index and low angle of extinction in reference to the twin- 

 plane trace. The sections are less clear than is usual for 

 Alderney examples, for cloudy blotches of chlorite abound. 

 These blotches sometimes scatter through the felspar in 

 roughly parallel rows. The felspar shows evidence of shear, 

 some repeated twinning exhibiting wavy curves, some show- 

 ing complete curving over, giving micro-anticlines. The 

 movement seems to give more bending than fracturing, an 

 argument for an action predominantly thermal. 



Evidence of orthoclase is not good, but it is undoubtedly 

 present. The green mineral with black iron oxide bandings 

 is chloritie, with sufficient indications in a large number of 

 cases to identify it as a development from hornblende. 

 Some fairly good dingy-green hornblende still remains, form- 

 ing outside portions of chloritie matter. 



Biotite is very rare, probably derived, and greenish 

 instead of with normal colouring. 



The rock suggests partial fusion under pressure of a 

 normal diorite, which contained originally soda-lime felspar 

 and hornblende. The pressure seems due to neighbouring 

 dyke intrusion. 



Altered Diorite from Slickensided Fault-Plane. 

 Faulting has produced a sap-green material, yellowish in 

 section." The mineral, though showing hornblende cleav- 

 age, has a hieh refractive index, is broken into numberless 

 grains and particles, shows second order polarisation colours, 

 nr»H a/onears to be derived epidote. Some sections show a 

 gradation from preen hornblende to obvious epidote. Others, 

 nHV>oiio-V> b^vinp- tvoicpl cleavage directions, have but little 

 colour, and the form of augite is evident in places, with high 

 polarisation colours and the appearance of a roughened sur- 



