338 MISS C. A. RAISIN ON THE LOWER LIMIT OE 



" slate." The edge shown in the slides is often quite even, but in 

 some specimens it exhibits irregularities due to intruding tongues 

 of greenstone or to included fragments of felsite*. At all the 

 junctions examined the groundmass is semi-opaque, with small dull 

 granules and incipient crystals. Specimens taken at intervals of a 

 lew inches show this structure passing into a rock containing small 

 augites, which, however, are clear and well formed ; and from this 

 condition we trace a gradual passage into the coarser dolerite. 

 Fibrous, actinolitic, and chloritic or micaceous aggregates have 

 formed ; and in all the examples lath-shaped felspars occur, which 

 have been replaced by a minute crystalline mosaic ; this is some- 

 times interrupted along a dark central line, which no doubt marks 

 the original twinning-plane. Mr. Blake states that " chiastolite " 

 occurs m the so-called " slate." I cannot find that mineral or any- 

 thing which could possibly be mistaken for it, unless it be these 

 altered felspars. The large felspars of the coarser dolerite show the 

 beginning of a similar change, which is common enough in examples 

 from many districts. 



In the Bryn-Efail quarry the rock in contact with the greenstone 

 along most of the section is undoubtedl}* a felsite, and not a grit as 

 marked on Mr. Blake's plan ; but at the southern end the rock does 

 present more difficulty, even when a micro^^copic examination is made. 

 The slides include rounded fragments, not unlike those in a grit, 

 but, after careful study, I am of opinion that the rock is reall}' the 

 felstone, modified by crushing f. Moreover, we might fairly expect 

 that, if a grit occurred with a southward strike, it would be trace- 

 able beyond the road ; here, however, I could not find it. But even 

 if the slides near the junction do represent a grit (and not the 

 modified igneous rock, as I believe) this must have been formed from 

 fragments of the felsite, and therefore (unless a pyroclastic rock, 

 which it hardly resembles) could not be of earlier age than it. 

 ISo that, in either case, Mr. Blake's argument is invalidated. 



At the south end of the quarry there is a second kind of felstone, 

 associated with that already described. This is pale grey and com- 

 pact, but without porphyritic quartz or felspar. The possibility of 

 a separation during fiow, in a mass still plastic, occurred to me as 

 an explanation. Prof. Bonney, however, suggested that probably 

 the junction marked the intrusion of one felsite (apparently the 

 compact rock) into another, though the two rocks might not difi'er 

 much in age. This suggestion proved to be correct, for, at a subse- 

 quent visit, I traced the compact felsite as a branched dyke, pene- 

 trating the porphyritic quartz-felsite in veins of varyiug width. 



Thus the Bryn-Efail quarry fails to provide the proof for a revo- 

 lution in our ideas as to the age of the Llyn-Padarn felsite. 



* These form, I suppose, the felsite running ' in veins into the slate 'de- 

 scribed by Mr. Blake. 



t Some parts clearly exhibit pressure-structures, and suggest that other 

 specimens, which are more difficult to distinguish from a grit, consist also of 

 felstone subsequently crushed. Some of the apparent fragments are probably 

 llrmer parts of the felsite, which escaped modification. Such parts may be 

 recognized in a slide cut from the heart of the mass of felstone, and were pro- 

 bably due to a want of homogeneity in the original lava. 



