284 PEOF. A. H. COX AND ME. A. K. WELLS OX THE [vol. Ltxvi,. 



more basic than those at the top (section, fig. 4, p. 289). The 

 main intrusion of acid material in its turn is stratigraphically above 

 the more basic intrusions. But, in contrast to the others, the 

 larger sill shows no stratiform variation according to density. In 

 its case both lower and upper contacts are composed of thin zones 

 of basic material ; the bulk of the mass, however, is uniformly acid 

 in composition, and shows but little variation in the field. Since 

 this intrusion is so much larger than the others, and its rocks are 

 so distinctive in the field, Ave shall describe it separately under the 

 term ' Crogenen granophyre ', and return later to the more basic- 

 granophyres. 



(i) The Crogenen Acid Granophyre 

 (or Microgranite). 



The main outerop stretches from. Llynau Crogenen on the west 

 to (xelli-llwyd on the east, a distance of 3| miles, with an average 

 width of nearly half a mile. It belongs to a mass which is sill- 

 like in habit, and is accordingly repeated on the south by the- 

 G-wernan Fault. The southern outcrop is now separated into- 

 two portions : a small eastern one near Penrhyn-gwyn, and a much 

 larger western portion below Tyrau-mawr and extending for three- 

 quarters of a mile bevond the south-western boundary of the map- 

 (PI. XX). 



The sill, therefore, spreads approximately along the strike for a 

 distance of over 5 miles. In the direction of dip it can be followed 

 for nearly 2 miles, and how much farther it may extend in that 

 direction it is impossible t© say, as a little beyond the southern 

 boundary of the map the steep dip carries it deep underground, so 

 that its further prolongation is everywhere concealed by overlying- 

 rocks. 



It will be noticed that, in its course from east-north-east to- 

 west-south-west, the intrusion gradually cuts across all the sedi- 

 mentary strata, from the Tremadoc Slates to the Lower Basic- 

 Series. The transgression is continued beA^ond the south-western 

 boundary of our map (PL XX) and the intrusion cuts into slates 

 well above the Lower Basic Series. The southward transgression 

 is also brought out by the fact that, whereas on Grelli-llwyd, north 

 of the G-wernan Fault, the sill is in the Lower Acid Series below 

 the Bryn Brith Beds, at Penrhyn-gwyn, on the south side of the 

 repeating fault, the intrusion is above the Bryn Brith Beds and 

 in the Crogenen Slates. The westward transgression is shown by- 

 the reappearance of the Penrhyn-gwyn sill at still higher strati- 

 graphical levels on the slopes of Tyrau-mawr. 



The sill-like habit of the intrusion can be inferred from the 

 manner in which its outcrops are repeated by the Gwernan Fault, 

 and also from other considerations. Actual rock-exposures of the 

 sill margins are not common, owing to the fact that the grano- 

 phyre almost invariably occupies lower ground than the invaded 

 rocks whether sedimentary or igneous, and according^ junctions 



