32 PROF. T. G. BON:N'Er AXD THE EET. E. HILL : [Feb. igi2f 



was described by Mr. Hill in his paper ou this island/ to which 

 Prof. Bonney contributed some petrographical notes. 



At that date the authors still assumed gneiss to be, as thev 

 had been taught in their youth, the extreme result of metamor- 

 phism in a sedimentary rock, though the latter of them called 

 attention to the very frequent signs of mechanical disturbance. 

 On their joint visit to these islands in lb88, they were convinced 

 that the foliation in the gneiss of Guernsey was a result of pres- 

 sure, and that the rock had been originally granite more or less 

 porphyritic — thus being almost everywhere a variety of what is 

 now called ' augen-gneiss," a conclusion announced by Mr, Hill"^ 

 in the following year. 



This visit and a passing glance in 1891 showed that, making 

 the necessary modifications required by the new interpretation 

 of the ' gneissic ' structure, the statements in Mr. Hill's first 

 paper were substantially correct, so it may suffice to say that 

 porphyritic felspars, which sometimes are about a couple of inches 

 in diameter, were almost always constituents of the original 

 granite.^ These and most of the smaller felspars in the matrix are 

 usually red, but the rock is locally greyish -white : the alteration 

 in colour being probably due to the change, in disseminated minute 

 granules, from magnetite to haematite. Biotite has been present, 

 but it is more generally altered into a chloritic mineral. 



The foliation, which is, of course, at right angles to the pressure, 

 is roughly north and south. It is the same at St. Malo, and near 

 Coutances is between this direction and north-north-east, while 

 about Cape La Hague it is north-east and south-west. These 

 indicate very ancient (probably pre-Cambrian) movements, almost 

 at right angles to the widely-extended Armorican folding. 



II. The Dioeites. 



This rock is now much more largely quarried than when 

 Mr. Hill was at work in Guernsey, the excavations being more 

 numerous and carried to a greater depth. The latter change, 

 however, does not facilitate examination, as the floor of the pit 

 often can only be reached by ladders, not easy of access, and the 

 quarry, when abandoned, is before long converted into a pool. In 

 such cases we had to be content with a view from above, and with 

 an examination of the heaps of material which were awaiting 



1 Q. J. G. S. vol. xl (1884) p. 404. 



2 E. Hill, ' The Eocks of Alderney & the Casquets ' Q. J. G. S. vol. xlv 

 (1889) p. 389._ 



^ It is possible that further scrutiiij- may show that there is more than one 

 mass. Mr. Hill, during his visit in April 1911, found a variety, bearing some 

 superficial resemblance to a diorite, in a quarry opposite Eocquaine Castle. 

 A slice shows it to be holocrystalline ; the quartzes, composite and not very 

 abundant ; felspars, rather irregular in outline, a considerable amount plagio- 

 clase ; decomposition, shown by minute white micas, setting in; biotite, fairly 

 plentiful, composite, a little altered, containing between its cleavage-planes 

 rather elongated crystallites (? sphene). So the rock is a gneiss that has- 

 undergone pressure-m odification . 



