NEIGHBOURHOOD OF SARN, CAERNARVONSHIRE. 455 



below the signal-staff, follows the trend of the hill-side in a north- 

 esasterly direction, and crosses over the ridge, where it begins to 

 sink towards its northern extremity. The topmost banks are 

 overlain by the hornblende-diabase, which forms the greater part of 

 the hill, and hornblende-diabase also occurs at the very base, thus 

 intervening between the picrite and the subjacent strata. 



In the field and in hand-specimens the appearance of this hand- 

 some rock is very striking. Most conspicuous is hornblende in 

 large black crystals with lustrous cleavage -planes. On closer 

 examination these planes are seen to be studded with rounded, dull 

 spots, which represent grains of olivine more or less serpentinized. 

 Besides this, flakes of a golden-brown mica are often plentiful, lying 

 on the cleavage-planes of the hornblende, and some parts of the 

 rock show white crystals of felspar. 



The microscope shows the original minerals of the hornblende- 

 picrite to be magnetite, olivine, felspar, augite, hornblende, and 

 biotite : among the secondary products are magnetite, serpentine, 

 hornblende, biotite, asbestos, a mineral of the chloritoid family, 

 calcite, and dolomite, with rarely an aggregate similar to that which 

 has been named saussurite. 



Original magnetite is not common ; it sometimes occurs in cubes, 

 and is the earliest-formed constituent. Picotite has not been 

 observed. 



Olivine is always one of the most abundant minerals present ; it 

 rarely shows crystal-contours, and is almost always in rounded 

 grains imbedded in augite or hornblende. Twinning is rare. The 

 grains sometimes show fissures corresponding to the two pinacoidal 

 cleavages, but more commonly are traversed by irregular cracks. 

 Under a high magnifying-power some of the olivine shows flat 

 rectangular cavities or " negative crystals " containing dendrites of 

 magnetite, identical with those figured by Professor Judd in a 

 picrite from the Isle of Eum*. As the Penarfynydd rock is 

 probably of Upper Cambrian (Bala) age, its resemblance to a 

 Tertiary rock, extending to such minute details, is a point of some 

 interest. The conversion of the olivine into serpentine is seen in 

 every stage from fresh grains of olivine to complete pseudomorphs. 

 The process begins along the fissures with the separation of mag- 

 netite dust, which, when plentiful, collects in clotted granules and 

 strings. Serpentine is next formed on the borders of the fissures in 

 fibres perpendicular to the walls. This serpentine is doubly 

 refracting and apparently uniaxal : the remaining kernels of olivine 

 are then gradually converted into serpentine, which either shows a 

 confused structure or is sensibly isotropic. Sometimes, as a last 

 stage, there is a seeming reabsorption of the deposited magnetite as 

 described by Wadsworthf. Irregular fissures sometimes radiate 

 from the altered grains, traversing the surrounding minerals, and 

 these fissures are injected with serpentinous matter : they may be 



* Quart. Journ. Geol. Soc. toI. xH. p. 385, pi. xii. fig. 5 (1885). 

 t ' Lithological Studies,' p. 172, &c. (1884). 



