File:The Quarterly journal of the Geological Society of London (13937107491).jpg

250

 MISS 
 C. 
 A. 
 RAISIN 
 OX 
 SOME 
 

belonging

 to 
 that 
 formation; 
 but 
 the 
 nearest 
 fossiliferous 
 outcrop 
 

recorded-

 is 
 distant 
 two 
 or 
 three 
 miles. 
 The 
 country 
 around 
 is 
 

drift-covered,

 but 
 I 
 found 
 some 
 indications 
 of 
 stratigraphical 
 position 
 

near

 the 
 streamlet 
 which 
 flows 
 to 
 Llym-gwyn. 
 Felsite 
 of 
 the 
 

usual

 type 
 occurs 
 close 
 by 
 the 
 cottages, 
 although 
 further 
 south, 
 just 
 

beyond

 the 
 railway, 
 is 
 one 
 outcrop, 
 which 
 seems 
 probably 
 an 
 ash, 
 

consisting

 of 
 broken 
 felstone 
 and 
 some 
 slate. 
 Walking 
 northward 
 

for

 about 
 thirty 
 yards, 
 along 
 the 
 path 
 east 
 of 
 the 
 stream, 
 we 
 pass 
 

over

 a 
 small 
 exposure 
 of 
 a 
 peculiar 
 black 
 and 
 white 
 rock, 
 which 
 

has

 a 
 squeezed 
 look. 
 The 
 milk-white 
 fragments 
 of 
 felstone 
 exhibit, 
 

under

 the 
 microscope, 
 traces 
 of 
 banding, 
 possibly 
 fluidal 
 ; 
 they 
 are, 
 

at

 places, 
 spherulitized, 
 and 
 contain 
 abundant 
 microliths 
 or 
 globulites. 
 

Slat.

 T 
 fragments 
 are 
 entangled, 
 which 
 are 
 often 
 rounded, 
 and 
 some 
 

are

 beginning 
 to 
 crack 
 along 
 curving 
 boundaries. 
 This 
 rock 
 seems 
 

to

 be 
 a 
 volcanic 
 agglomerate, 
 which 
 has 
 become 
 schistose 
 from 
 

pressure.

Along

 the 
 next 
 200 
 yards, 
 well-banded 
 ashy 
 and 
 slaty 
 beds 
 occur 
 

here

 and 
 there, 
 in 
 which 
 I 
 found 
 a 
 few 
 small 
 fossils. 
 These 
 were 
 

several

 squeezed 
 examples 
 of 
 Trinudeus 
 concentricus, 
 small 
 specimens 
 

of

 Leptcena 
 sericea, 
 a 
 distorted 
 Orthis 
 elega?itula, 
 and 
 other 
 young 
 

forms

 of 
 Ortliis 
 (sp. 
 ?). 
 This 
 assemblage 
 has 
 a 
 Bala 
 facies, 
 and 
 Mr. 
 

Etheridge,

 who 
 very 
 kindly 
 identified 
 some 
 of 
 the 
 specimens 
 for 
 

me,

 gave 
 his 
 opinion, 
 from 
 the 
 fossil 
 and 
 lithological 
 characters, 
 

that

 the 
 strata 
 may 
 be 
 taken 
 as 
 belonging, 
 in 
 all 
 probability, 
 to 
 that 
 

formation.

 The 
 microscope 
 shows 
 quartz 
 and 
 plagioclase 
 felspar 
 in 
 

small

 angular 
 fragments 
 and 
 a 
 minute 
 secondary 
 mineral 
 ; 
 the 
 

meshwork

 of 
 dark 
 cleavage-planes 
 crosses 
 the 
 beds 
 almost 
 at 
 right 
 

angles.

Some

 30 
 yards 
 succeed, 
 occupied 
 by 
 a 
 felspathic 
 rock, 
 part 
 of 
 

which

 forms 
 two 
 small 
 craglets 
 overlooking 
 the 
 stream. 
 The 
 beds, 
 

although

 uniform, 
 are 
 well 
 marked, 
 dipping 
 about 
 ET.N'.E. 
 at 
 an 
 

angle

 of 
 45° 
 or 
 50°. 
 They 
 are 
 crossed 
 by 
 a 
 cleavage 
 similar 
 to 
 

that

 of 
 the 
 slaty 
 and 
 ashy 
 strata, 
 and 
 the 
 subsequent 
 pressure, 
 

which

 thus 
 modified 
 the 
 rock, 
 doubtless 
 caused 
 the 
 fracturing 
 of 
 

the

 included 
 felspar. 
 The 
 appearance 
 of 
 the 
 slide 
 is 
 suggestive, 
 as 
 

Professor

 Bonney 
 pointed 
 out 
 to 
 me, 
 of 
 a 
 very 
 porphyritic 
 rhyolite, 
 

now

 devitrified 
 and 
 crushed. 
 

The

 next 
 outcrop 
 is 
 of 
 a 
 compact 
 pale 
 grey 
 rock, 
 which 
 proves 
 to 
 

be

 a 
 felstone, 
 enclosing 
 small 
 fragments 
 of 
 slate, 
 quartz, 
 and 
 felspar. 
 

The

 fluidal 
 layers 
 are 
 much 
 contorted, 
 and 
 exhibit 
 a 
 deposit 
 of 
 a 
 

minute

 secondary 
 mineral. 
 

Thus

 the 
 Pen-y-chain 
 felsites 
 seem 
 to 
 pass 
 upward 
 into 
 a 
 series 
 

of

 agglomerates 
 and 
 lavas, 
 with 
 interbedded 
 slates 
 and 
 grits, 
 including 
 

Bala

 fossils. 
 If 
 the 
 igneous 
 strata 
 of 
 the 
 Snowdon 
 district 
 once 
 

extended

 over 
 the 
 Lleyn, 
 as 
 suggested 
 in 
 the 
 Survey 
 Memoir 
 *, 
 the 
 

Pen-y-chain

 mass 
 may 
 be 
 a 
 remnant 
 of 
 such 
 volcanic 
 accumu- 
 

lations.

The

 small 
 area 
 of 
 Careg-y-defaid 
 does 
 not 
 need 
 separate 
 detailed 
 

description.

 • 
 A 
 fine 
 fluidal 
 structure 
 in 
 a 
 porphyritic 
 rock 
 is 
 well 
 

 The 
 Geology 
 of 
 tfortk 
 Wales, 
 pp. 
 218,