part 3] LOWER PALiEOZOIC OF AKTHOG-DOLGELLEY. 



289 



02 » I - 



(J Ofl 



o ^^ 



>. 

 p S 4 



a S «e 



■Crogenen granophyre closely resembles the Buttermere and Enner- 

 ■dale granophyre. 1 The Welsh and the Lake-District intrusions 

 present other points of similarity, for both show similar marginal 

 rocks formed under similar conditions, both are intrusive into 

 Lower Ordovician strata approximately along the junctions of a 

 .slate series with a volcanic series, and, as will be shown later 



(p. 30-1), the intrusions 

 in both districts are pro- 

 bably of the same age. 



The minor intrusions 

 are separated one from 

 the other and from the 

 main sill by small thick- 

 nesses of Dolgelley or 

 Tremadoc Beds ; unlike 

 the main sill, they never 

 intrude into Ordovician 

 rocks. They may con- 

 veniently be styled the 

 Craig -y-Castell, the 



smaller and the larger 

 Tyn-y-llwyn, and the 

 Waen-Fechan intrusions 

 respectively. Detailed 

 references to their modes 

 of occurrence are unneces- 

 sary, since most of such 

 information can be ob- 

 tained from the map 

 (PL XX). 



As regards the composi- 

 tion of the rocks: (i) The 

 Craig-y-Castell intrusion 

 is in its lower half com- 

 posed of a basic grano- 

 phyric rock closely com- 

 parable with many quartz- 

 dolerites. The upper half 

 of the intrusion is com- 

 posed of a coarse-grained 

 markedly granophyric 

 rock less basic than that 

 in the lower half, but 

 distinctly more basic 

 than the typical Crogenen rock. It appears, in fact, to be similar 

 to, but perhaps somewhat more basic than, the well-known grano- 

 phyres of MarkHeld and Groby in Charnwood Forest ; accordingly, 

 the term markf ieldite may fittingly be applied to it (see below, 

 p. 300). 



1 E. H. Eastall, Q. J. G. S. vol. lxii (1906) p. 253. 



a 3 



u ? 



>» u 



" J 



^ "S 



S cS 







£3 •-< 



cp j£> 



p^ s 



'ft^ 



EH r° 



.S 1 



72 a; 



V ^ 



rH cp) 





a 



<B fl 



8 a) 



'an a> 



■H rfl 



ri aj 



id += 



3£ 



!■! 



II II 





00 ■* 



J3 ^ 





o S 





o <s 





h o 







. 



.s""S 



_o 



££ 



"oQ 



r^ a 





& tf 



9 £ 

 .2 a 



O H 





-2^ 







"5c -+^ 





cS c3 



■s « 



^ .2 



cS >-> 



+3 -rt 



OH 



^3 a 



^ e3 



be Zl 

 "3 cS 



S 03 



■ rt a 



^ a 



03 _0 



II II 



13/-+3 



£ 2 



iH ©3 



m i-s 



