294 PROF. A. H. COX AND MB. A. K. WELLS ON THE [vol. lxxvi, 



are developed. The most widely distributed type would, considered 

 by itself, be best described as a microgranite having a tendency to 

 graphic structure (PI. XVIII, figs. 1 & 2). Other specimens are, 

 however, typical granophyres with very beautiful and delicate 

 growths of micropegmatite, the exact arrangement of which 

 changes, as usual, with the degree of coarseness, becoming almost 

 centric in the fine-grained types, and so giving a patchy pseudo- 

 spherulitic appearance when viewed under the microscope. Graphic 

 structures are usually best developed in specimens of finer grain, 

 whereas the microgranitic type is to be found in specimens of 

 rather coarser grain, though some exceptions to this rule do occur 

 (PI. XVIII, fig. 4). The most markedly granophyric rocks are 

 found in comparatively narrow zones towards both the upper and 

 the lower margins of the sill, though not actually on the margins 

 themselves (PI. XVIII, fig. 3) : for, as the outermost zones are 

 reached, the rocks become distinctly basic, and all traces of micro- 

 graphic structure disappear (PI. XIX, fig. 3). Thus in part, at anv 

 rate, the development or otherwise of the micrographic structure 

 depends upon differences in chemical composition, which itself is 

 influenced by the position of the rock in the sill. 



A further cause of these small-scale variations in the relative 

 development of graphic structures, according to the chemical 

 composition of the rock, is to be found in the gradual destruction 

 of basic patches which represent cognate xenoliths derived from the 

 basic marginal modifications. Such patches of more basic material 

 persist for a long time and for long distances into the sill. The 

 distinction between patches and ground-mass naturally becomes less 

 marked towards the interior, until eventually mere suggestions onlv 

 of patchiness remain, showing slight variations in the character of 

 the quartz-felspar intergrowth. 



Chemical composition. — The Crogenen granophyre has not 

 been analysed ; but the rock may be expected to compare closely 

 with the petrologically similar rock on Cader Idris, of which an 

 analysis was recorded by Prof. G. A. J. Cole & Mr. A. V. Jennings, 

 and is quoted below along with the results given by other similar 

 rocks of North Wales and the Lake District. 



I. II. III. 



Si0 2 7279 75-02 



AU0 3 13-77 12-88 



Fe 2 3 3-32 2"89 



FeO — — 



MnO trace 



MgO 



CaO 



Na 2 



K 2 2-99 5-03 3-56 



Loss on ignition ... 

 Other constituents 



0-62 



0-32 



1-94 



1-17 



4-12 



3-28 



2-99 



5-03 



1-08 



0-60 



100-63 



101-19 



~i 



^^^— 



T. H. 



L. W. 



Holland. 



Fulcher 



IV. 



V. 



71-60 



80-12 



13-60 



10-68 



2-40 



0-06 



— 



0-49 



— 



0-05 



0-21 



063 



2-30 



1-01 



5-55 



3-44 



3-53 



1-42 



0-70 



1-08 



— 



1-42 



99-89 



100-40 



G. 



H. F. 



Barrow. 



Harwood. 



