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NODULAR

 EELSTONES 
 OF 
 THE 
 LLEYN. 
 255 
 

evidence

 of 
 the 
 process 
 of 
 silicification, 
 which 
 is 
 illustrated 
 in 
 all 
 the 
 

quartz-filled

 vesicles, 
 and 
 in 
 changes 
 superinduced 
 in 
 the 
 matrix 
 of 
 

the

 rocks. 
 

III.

 Nodular 
 Structures. 
 

In

 the 
 Survey 
 Memoir 
 the 
 occurrence 
 of 
 these 
 structures 
 is 
 

noted*,

 but 
 no 
 details 
 are 
 given 
 of 
 their 
 various 
 forms. 
 

1.

 Perlitic 
 /Spheroids 
 and 
 Concentric 
 Structures 
 in 
 Nodules. 
 — 
 

Certain

 of 
 the 
 nodules 
 appear 
 to 
 be 
 the 
 result 
 of 
 simple 
 contraction 
 f 
 , 
 

since

 they 
 are 
 lithologically 
 similar 
 to 
 the 
 rock 
 which 
 includes 
 them 
 ; 
 

in

 fact, 
 the 
 boundary 
 may 
 be 
 developed 
 only 
 on 
 a 
 weathered 
 surface. 
 

In

 a 
 slide 
 from 
 one 
 example 
 there 
 is 
 no 
 difference 
 in 
 the 
 closely 
 sphe- 
 

rulitic

 ground-mass 
 of 
 nodule 
 and 
 of 
 rock, 
 except 
 a 
 slightly 
 darker 
 

staining

 in 
 the 
 neighbourhood 
 of 
 the 
 boundary-crack. 
 

Some

 of 
 the 
 masses 
 found 
 towards 
 the 
 east 
 of 
 Careg-y-defaid 
 seem 
 

to

 be 
 due 
 to 
 flow-brecciation 
 ; 
 they 
 are 
 subangular, 
 varying 
 from 
 

about

 one 
 inch 
 to 
 three 
 feet 
 across, 
 and 
 appear 
 firmer 
 and 
 more 
 

silicified

 than 
 the 
 somewhat 
 schistose 
 matrix. 
 Many 
 of 
 them 
 are 
 

slightly

 elongated 
 in 
 the 
 direction 
 of 
 the 
 lamination, 
 and 
 are 
 marked 
 

by

 a 
 parallel 
 fissuring, 
 along 
 which 
 the 
 ground-mass 
 often 
 penetrates 
 

into

 the 
 nodules. 
 

One

 of 
 these 
 brecciated 
 masses, 
 roughly 
 rhomboidal 
 in 
 shape 
 and 
 

rather

 larger 
 (about 
 six 
 feet 
 by 
 three 
 and 
 a 
 half 
 feet), 
 is 
 full 
 of 
 

half-inch

 spheroids, 
 which 
 exhibit 
 sintery-looking 
 concentric 
 shells, 
 

recalling

 the 
 description 
 given 
 by 
 Zirkel 
 of 
 spherulites 
 thus 
 weather- 
 

ing

 %. 
 Microscope 
 sections 
 show 
 a 
 kind 
 of 
 granular 
 micrographic 
 

growth,

 with 
 a 
 partial 
 attempt 
 at 
 a 
 radial 
 arrangement, 
 the 
 concentric 
 

hollows

 apparently 
 developing 
 within 
 the 
 less 
 fibrous 
 zones. 
 If 
 an 
 

external

 part 
 of 
 the 
 lava 
 had 
 been 
 broken 
 up 
 in 
 the 
 flow 
 it 
 would 
 

probably

 be 
 vesicular 
 ; 
 and 
 it 
 is 
 conceivable 
 that, 
 under 
 the 
 altered 
 

conditions,

 vapours 
 which 
 were 
 contained 
 might 
 expand 
 to 
 form 
 

the

 irregular 
 concentric 
 hollows, 
 somewhat 
 in 
 the 
 way 
 described 
 by 
 

von

 Eichthofen§. 
 We 
 might 
 suppose, 
 however, 
 from 
 the 
 partial 
 

and

 discontinuous 
 spherulitic 
 structure 
 which 
 borders 
 the 
 internal 
 

cavity

 and 
 the 
 exterior 
 boundary, 
 that 
 the 
 radial 
 growth 
 was 
 pos- 
 

sibly

 connected 
 in 
 its 
 origin 
 with 
 contraction 
 towards 
 a 
 centre, 
 and 
 

that

 planes 
 of 
 weak 
 cohesion 
 might 
 have 
 thus 
 arisen, 
 as 
 Professor 
 

Bonney

 suggested 
 in 
 explanation 
 of 
 the 
 spherulitic 
 felsite 
 of