PICRITE NEAR PEN-Y-CARNISIOG, ANGLESEY. 



139 



merly a true augite. The extinction-angles are generally less than 

 20° ; but in one slide are two crystals which, though dichroic, give 

 angles of about 30°. The larger crystals of the Schriesheim picrite 

 agree with augite in their feeble dichroism and general appearance ; 

 but the results of several measurements of the extinction-angle are 

 less than 20° in the case of the largest crystal, while in a smaller 

 one they are over 30° ; and a very considerable quantity of horn- 

 blende, similar to the varieties (a) and (6), is present in the body of 

 the slide. A little olivine has escaped change ; and the structure of 

 the rest is rather more characteristic. Still, except that a mica is 

 decidedly more abundant in the Schriesheim than in the Pen-y-Car- 

 nisiog rock, the main difference appears to me to be, that the latter 

 has undergone more alteration than the former, so that it, too, has 

 once been a true picrite. 



Another rock is very abundant in boulders in this district of 

 Anglesey, which, at first sight, has some resemblance to the picrite, 

 though less porphyritic in structure. Closer examination, however, 

 shows that felspar is always a constituent of this, though it is often 

 not very conspicuous. I have examined a specimen microscopically, 

 and find it to consist of a green hornblende, an altered felspar, a 

 brown mica more or less changed, apatite, and a chloritic mineral. 

 The hornblende and felspar are rather irregular in external form : the 

 latter is almost wholly replaced by microlithic products ; but one or 

 two grains still retain sufficient traces of their original structure to 

 show they have been plagioclastic. It is somewhat singular that a 

 rather similar rock, but with more brown mica and better-preserved 

 felspar, occurs at Schriesheim, within a short distance of the picrite, 

 also intrusive in the granite. This is named a Labrador-diorite by 

 the German petrologists. 



The only other instances known to me of the occurrence of picrite 

 in the British Isles are two in Pifeshire, described by Prof. Geikie 

 in his excellent monograph on the Carboniferous Yolcanic Rocks of 

 the Pirth-of-Porth basin*, and one described by myself f, to which, 

 as I had at that time never examined a typical picrite, and had 

 failed to obtain a very clear notion of the rock from such descrip- 

 tions as I had seen, I did not venture to attach the name. In this, 

 however, olivine (still very fairly preserved) is the dominant mi- 

 neral ; so that it comes nearer to a normal peridotite. The rock was 

 collected many years ago by Professor Sedgwick, near Penarfynydd, 

 in the Lleyn peninsula ; and Mr. E. P. Tawney, who lately visited 

 the locality expressly to search for it in situ, failed to find it, and 

 believes that the specimen must have been taken from a boulder. 

 Here also, as described by Mr. Tawney, are olivine-diabases and 

 hornblendic diabases. 



I have ventured to draw especial attention to this Anglesey spe- 

 cimen, in the hope that some geologist may succeed in discovering 

 a like rock in situ. As the picrite just described is so uncommon 

 and of so marked a character, we might assume with much confi- 



* Transact. Hoy. Soc. Edinburgh, vol. xxix. p. 437. 



t In a paper by Mr. E. B. Tawney, Geol. Mag. Dec. 2, vol. yii. p. 208. 



