part 3] lower palaeozoic of arthog— dolgelley. 297 



■complete resorption of the basic material. The finer-grained 

 xenoliths (type 1) usually retained definite outlines ; whereas the 

 coarser-grained xenoliths (type 2) were more readily corroded by 

 acid material, and hence have lost their originally definite shapes, 

 with the result that eventually the xenolithic structure degenerates 

 into a mere patchiness [C 221]. Sometimes, instead of the irregular 

 patchiness. a definite banded structure is developed, as a result of 

 flow of the hybrid magma [C 215 & C 211]. 



Eventually, as the absorption becomes increasingly complete, 

 the excess of dark minerals disappears, and the only sign that one 

 is dealing with a more basic type than the normal granophyre is 

 the presence of irregularly-defined patches containing a greater 

 proportion of felspar and less quartz, the felspar being at the same 

 time more idiomorphic than is normally the case in the granophyre. 

 Such felspathic patches persist for some time, even in the main 

 mass of the granophyre sill, as stated on p. 294*. The ill-defined 

 patches may be accompanied by more sharply-defined xenoliths of 

 chilled material (tvpe 1) set in a rock that is otherwise a normal 

 granophyre [C 228, PL XVIII, fig. 4]. 



It should be noted that the most basic types (1 & 2) are not 

 invariably present along the margins of the main intrusion ; the 

 actual marginal rocks are then of the slightly more acid type (3). 



(2) The minor granophyric intrusions. — It will be con- 

 venient to describe these rocks in order from basic to acid. The 

 most basic type is seen in three of the intrusions : namely, in the 

 lower halves of the Craig-y-Castell and the smaller Tyn-y-llwyn 

 masses, and forming the whole of the Waen-Fechan sill. Rocks 

 of this tvpe are dark green, thoroughly basic in composition, and 

 usually rather fine-grained. In many respects they resemble 

 quartz-dolerites, but they are marked off from such rocks by reason 

 of their fine-grained texture, and by the fact that the principal 

 dark minerals are hornblende and biotite, not augite. Still more 

 closely do they resemble some of the rocks found in the basic 

 margin of the Crogenen granophyre, differing from these principally 

 in having a somewhat coarser and more nearly subophitic texture 

 (compare PL XIX, figs. 1 & 2). 



A specimen from the quarry near Waen-Fechan Farm, at the 

 western end of the Waen-Fechan intrusion, shows felspars to be 

 the largest crystals present [C 234, PL XIX, fig. 1]. They 

 are quite idiomorphic, somewhat columnar in habit, and usually 

 are slightly turbid through decomposition (which, however, has not 

 proceeded far enough to disguise their optical properties), and 

 they appear to be near albite in composition. Dark minerals are 

 abundant and varied ; fibrous green hornblende is prominent, and 

 a pleochroic chlorite is also plentiful, frequently enclosing rosettes 

 and isolated crystals of a yellow epidote rich in iron. Angular 

 interspaces between the larger idiomorphic felspars are often 

 occupied by a confused network of actinolitic hornblende, set in 

 a matrix of clear felspar-granules. A strongly-pleochroic biotite 



