16 noivard Fox — On a Soda Felspar Rock — 



cylindrical cavities, which have been filled with rusty-coloured 

 material containing crystallized quartz. This rusty material occa- 

 sionally weathers out as nodules 8-10 ram. in diameter, projecting 

 from 10-15 mm. beyond the white surface of the rock. The rusty 

 material has, however, in most cases worn away faster than the 

 matrix, and left the rock dotted thickly with conspicuous rusty- 

 coloured depressions. In some places the rusty material forms, 

 lenticles and fills up cavities between the beds of the rock. 



Dr. Hinde examined a section of this rock for me, and thus refers 

 to the white weathered portion : " The white crust appears as if of 

 a flaky character and forming numerous extremely thin layers. 

 "Whether these represent layers of original bedding seems doubtful, 

 but it is possible that it might be so." 



As we proceed in a north-westerly direction from "c" we pass 

 over exposures which show that the igneous rock was posterior to 

 and intrusive in the soda rock, the beds of the latter being bent, 

 disordered, and cut across, as seen in Fig. 2, which with Figs. 1 

 and 3, are taken from the original sketches of Sir Archibald 

 Geikie, whom I had the pleasure of accompanying to the spot. 



Soda rock 



ThrusT 



cru/itweyreenst^e"/ /_/ ^^S^^^^^ ^.lane 



D/AjAS he: no 



Fig. 1. Sketch of junction of greenstone and soda rock on Southern Cliff of Dinas 

 Head, showing movement since intrusion of greenstone. 



That there has been considerable movement since the intrusion 

 of the greenstone is shown by Fig. 1, where a thrust-plane is 

 indicated. In some places the rocks have been shattered and re- 

 cemented by quartz, and quartz-veins run through both the igneous 

 and the soda rock, being more numerous in the latter. In one place 

 a mass of soda rock 15 feet long is enclosed in the greenstone. 



Further west we find the cliff shattered with faults, and at "d" 

 the soda rock and the dark compact slates contain round and oblong 

 rusty-coloured concretions of dolomitic character, from an inch to 

 several feet in diameter. The concretions frequently have con- 

 centric structures around them. Just above "d" there are dolomitic 

 bands 4 feet thick, as shown in Fig. 3. Here a dolomitic band " 6," 

 4 feet thick, is interbedded with dark cherty slates " a," in which 

 are concretions and lenticles of dolomite, many of the concretions 

 having concentric structure. The thrust-plane " c " has a dolomitic 

 fault breccia lying in cakes about' it. A section of a concretion is 



