596 PROP. T. G. BONNET AND MISS C. A. RAISIN ON [Nov. 1 894, 



that next to the felsite, which lithologically it seems to resemble. 

 In that case the fault probably would be as great as that north of 

 the section east of tbe lake, while the throw of one to bring in the 

 Purple Slate would have to be rather greater. 1 



North-east of the lake the following seems to be the general 

 succession of the beds : — (a) a lower conglomerate mainly derived 

 from the quartz-felsite, often containing also some pebbles of quartzite 

 and granitoid rock (3 in fig. 2, p. 590) ; (b) a grit of similar 

 materials (4 in fig. 2) ; (c) coarse strata, including fairly numerous 

 fragments of somewhat basic igneous rock, 2 as in the ' rain-spot 

 breccia, the overlying conglomerate, and other pebbly bands (5 and 

 6 in fig. 2) ; associated grit of similar composition interbanded with 

 argillite (7 in fig. 2) ; (d) green banded finer grit, halleflinta, and 

 argillite and purplish fine-grained rocks (8 in fig. 2), leading up to 

 the Purple Slate. These last indicate the beginning of the con- 

 tinuous quiet deposition which characterizes so much of the 

 Cambrian period. They appear to be associated with the green 

 grits containing coloured quartz-grains, as already described. 



In lithological character the third (or southern) mass of con- 

 glomerate in the railway-section seems to be similar to the first or 

 northern. As already described, the dips above the third con- 

 glomerate indicate an anticlinal, and thus the grits, conglomerate, 

 and banded scries on the north may be taken as higher beds. If so, 

 they are probably cut out to the north-west of the synclinal of green 

 banded rocks, and to the south of the third conglomerate, by two 

 faults, both of which are admitted by Mr. Blake (see F 2 and F 3 

 in our map, p. 594). Probably the fault between the felsite and 

 the conglomerate (F 1 ) is small, but that along which the greenstone 

 is intruded on the south of the western conglomerate (F 2 ) is greater, 

 for it apparently cuts out beds 4, 5, 6 (figs. 2 and 4), perhaps a little 

 of the upper part of 3, and some of the lower part of 7. 



We believe that the anticline of the railway-section is indicated 

 to the N.E. at various spots 3 towards the Moel Goronwy felsite, and 



1 Possibly the first might be 400 feet, the second (still more difficult to esti- 

 mate) 600-700 feet. But the fault at Nantlle is described by the Geological 

 Survey as throwing the slate directly against the quartz-felsite. 



2 Probably similar to those mentioned by Sir A. Geikie (Quart. Journ. Geol. 

 Soc. vol. xlvii. 1891, Pres. Addr. Proc. p. 96). We do not, however, see that 

 there is proof of the contemporaneous ejection of these. It is difficult to con- 

 ceive them as the ejectamenta of expiring volcanic eruptions, since they are 

 different from any of the larger masses of igneous rock in the neighbourhood — 

 unless, indeed, certain more distant rocks of similar composition (such as those 

 in the Lleyn) could be proved to belong to this age. 



3 At more than one place a close resemblance can be traced to the banded 

 grit south of the third conglomerate on the railway, dipping south-easterly, and 

 other dips to the north-westerly point are exposed. Between the two roads 

 which join to the eastward at the School, a curious purplish rock occurs, and 

 also a more normal-looking purple felsite (possibly a dyke — we have not 

 examined its relations). The former shows in places a clean-welded junction 

 with a greyish felsite and exhibits a structure like a flow ; it even runs in 

 among fragments of the felstone. Both rocks have been affected by subsequent 

 pressure. The purplish rock rather reminds us of a specimen described by one 

 of us from Baron Hill Park, near Beaumaris (T. G. Bonney, Quart. Journ. 

 Geol. Soc. vol. xxxix. 1883, p. 472). 



